macOS

macOS has many privacy and security settings that can be turned off or tuned, reflecting in part how you interact with a computer versus a mobile device. Let’s start with System Settings > Privacy & Security, which collects several controls and options. I also mention settings in Users & Groups, and Lock Screen, and in the Keychain Access utility, that deserve a quick look.

There is more on FileVault separately.

Unlock the pane or setting

Apple requires you to prove your identity when accessing more sensitive elements of System Settings. First, click a setting, like changing “Allow applications from” to App Store. Now:

• Touch ID: If you have Touch ID, you’re prompted for biometric authentication. You can also click Use Password to enter your account password, then click Unlock.
• No Touch ID: You’re prompted to type in the administrator password. Enter it and click Unlock.

Some settings, such as Users & Groups, prompt for your administrator password even when Touch ID is enabled.

With an Apple Watch, you can also unlock many settings by using a side button double-press. Go to System Settings > Touch ID & Password and enable your Apple Watch under “Use Apple Watch to unlock your applications and your Mac.”

That option also lets your Apple Watch unlock your Mac’s screen, as described below.

Require an unlock for all preferences

You can force yourself or users of the Mac to have to unlock all preferences before using them. Click the Advanced button in Privacy & Security, and then select or enable “Require an administrator password to access system-wide preferences.” Items like Sharing settings can otherwise be used without an administrator password.

Settings > Touch ID & Password and System Settings > Lock Screen offer two additional password-related features worth examining.

Adjust password preferences

In Touch ID & Password, the top of the setting says “A login password has been set for this user” if you set one. Click Change to update it.

You should always have a password set for your Mac, even if you’re the only person in your house, because a password to access your computer is a fundamental building block of security. If someone accesses your computer without your permission or steals your computer, you want at minimum this level of protection.

Once you change your password, you can’t use the old one again. While that might seem obvious, Apple added a grace period starting in iOS 17/iPadOS 17: change the passcode on an iPhone or iPad and you can still use the old one for 72 hours. Apple never added this to macOS, possibly for unknown security reasons.

In Settings > Lock Screen, select Immediately from the “Require password after screen saver begins or display is turned off” to, you know, do what it says on the label: force your Mac to lock when it goes to sleep, or when the screen saver activates and a period of time passes. You can also set it to wait for durations from 5 seconds to 8 hours.

How to sleep or lock your Mac

Apple offers lots of ways to let or force your Mac to go to sleep or initiate its screen saver. Here are several:

• Turn display off on…when inactive: In Settings > Lock Screen, you can set “Turn display off on [battery, power adapter] when inactive” to durations from 1 minute to 3 hours or Never. “Battery” and “power adapter” appear separately on laptops; only “power adapter” appears on desktops.
• Screen saver: Go to System Settings > Wallpaper, click Screen Saver, and use the “Start Screen Saver” pop-up menu. Choose a duration greater than Never for how long your Mac must be idle before the screen saver activates.
• Lock screen menu: Choose Apple menu > Lock Screen, and either the lock screen appears or the screen saver, depending on your settings. You can also press ⌘-Control-Q.
• Hot corner: Assign Lock Screen or Start Screen Saver to an action in System Settings > Desktop & Dock > Hot Corners. Each corner may have a different action assigned from a diverse list.

When you return, you can enter the password; or, on Macs with built-in or keyboard-based Touch ID enabled, unlock with Touch ID. See how to set up Touch ID and Face ID.

Touch ID may be disabled in certain circumstances for added security. See when biometric lockout happens.

You can choose Never, but I’d suggest…never doing this. Nearly everyone should pick Immediately or a short time duration—otherwise, every time you walk away from an active Mac, anyone with physical access can jiggle the mouse or touch the trackpad and access your stuff.

You can crank that up a notch if you’re at higher risk of physical access or theft: click the Advanced button in Privacy & Security, select “Log out automatically after inactivity,” and enter a duration in minutes. The Mac logs you out of your account after that period passes, raising the bar for a physical break-in. While the lock screen should be sufficient, background apps and other settings are in effect that could make your machine slightly more at risk than if it’s in a logged-out state.

If you have an Apple Watch, an iPhone, and a Mac signed in to the same iCloud account, you will have an additional option in System Settings > Touch ID & Password: “Use your Apple Watch to unlock your applications and your Mac.” Any Apple Watch that meets the preceding criteria appears in a list, each with its own switch. Enable one (or more) if you want your Mac to be automatically unlocked when it’s locked, you’re close by, and you’re wearing your watch. This setting also lets you unlock apps that require Touch ID via your Apple Watch, including the Apple Passwords app, and some system features.

Just tap the keyboard, keypad, or mouse as if you were jostling it to get it to show you a password dialog or a Touch ID prompt, and the Apple Watch and Mac have a conversation. Bluetooth and Wi-Fi must be enabled on the Mac for it to work.

Conversely, you might find unlocking your Mac with an Apple Watch is a security risk if you are subject to targeted attacks by a government or individuals.

Optionally, you can have a message appear on a lock screen. In Settings > Lock Screen, enable “Show message when locked,” click the Set button, enter a message, and click OK. This message can be useful if other people use the same Mac and you want to alert them as to why it’s idle.

I have my Mac in a day-lit basement office. Sometimes, I’ll be walking across our main floor, and my Apple Watch lights up that the Mac was unlocked a floor below!

Control what can be plugged in

Apple has options that can prevent peripherals, computers, and other accessories from connecting over built-in ports, like USB (Type-A and USB-C), Lightning, and SD Card, depending on your device. You can prevent peripherals from connecting via Lightning, Thunderbolt/USB, or the Smart Connector (for an iPad Smart Keyboard), or allowing a card to mount via the SD Card slot without permission.

Go to Settings/System Settings > Privacy & Security > Wired Accessories (iPhone/iPad)/Accessories (Mac). Modify the Allow Accessories to Connect setting. You have four options:

• Always ask
• Ask for new accessories
• Automatically allow when unlocked
• Always allow

In any case in which you have to grant permission, you must enter your administrator permission—Touch ID or Face ID, even when available, isn’t considered a high-enough bar.

Starting in macOS 26 Tahoe, you can also choose a security policy for accessories at startup. You can use this to prevent peripherals other than drives from interacting with the system before a macOS session has started.

First, restart into recovery mode; see how macOS protects its own system files. Next, choose Utilities > Startup Security Utility. Now, choose an option as above from the “Allow accessories to connect” menu.

With all that in mind, you might wonder: what should I open for use and what’s safe? The answer depends on whether your Mac has a public IP address. I advise always turning on only the services you need. A few are worthwhile, and how you configure them depends on your circumstances.

But you should have different considerations under these sets of circumstances:

• Privately addressed Mac on a network in which all users are trusted: If you’re by yourself or you trust your roommate and family to not attempt to access things they shouldn’t, you can enable what you like and just configure for general safety.
• Privately addressed Mac on a network in which not all users are trusted: If you share a network with other people—which could include kids or parents who may not be as safe or honest with you as you might like—consider locking things down further and using the fewest possible services.
• Publicly addressed Mac: You need to take more care to set passwords and configure limitations, and to consider whether to leave services active all the time or only when you know you or someone you trust needs them.
• High-risk individual or group: People who believe themselves at elevated risk shouldn’t operate any services on public addresses, and should think strongly about not running them even on privately addressed networks. It’s very easy for so-called Internet of Things (IoT) or “embedded devices” to be compromised. That can include DVRs, routers, smart home gear, and more. A subverted device on your local network can have the same access as anyone else on the network.

With that in mind, here are the items in System Settings > General > Sharing worth enabling or disabling. Configuration options appear when you click the info button next to a service.

Don’t allow all users

Several services let you selectively enable them. Apple offers a choice of “Allow access for” either “All users” or “Only these users” for most services. File Sharing has more fiddly options. I recommend setting access for Screen Sharing and any service starting with Remote to Administrators (often prefilled), or, better, selecting users or groups you’ve created. This limits exposure in the case of an intrusion that doesn’t gain direct access to your macOS account.

Screen Sharing

If you have a “headless” Mac (one without a monitor) that you use for various services, or machines in different parts of a house or office that you want to access remotely, screen sharing is handy.

I recommend deselecting two options in the Computer Settings dialog:

• Anyone may request permission to control screen: There’s no reason to allow this except momentarily if you want to allow someone to request access.
• VNC viewers may control screen with password: VNC is an outdated protocol that Apple’s screen-sharing technology extends with more security and features. There’s no good reason to keep this option turned on, particularly because VNC passwords are notoriously easy to crack.

File Sharing

The File Sharing service on a Mac offers fine-grained controls to choose which folders and volumes are accessible to which people, including the type of access: read/write, read-only, or write-only (to drop items into without seeing the shared item’s contents).

In configuring File Sharing, I recommend the following:

• Disable Full Disk Access if multiple people have accounts with administrator access to the Mac, unless you explicitly want them all to have remote access to all files on the volume.
• In the Shared Folders list, select items you don’t need shared and click the minus button.
• In the Users list for each shared folder in turn, click the pop-up menu and choose No Access for users and groups that can’t be removed, then select the user or group and click the minus button to remove those that can.

What you’re left with is a limited, tightly controlled set of folders and volumes that only people you have approved can interact with.

I often create a sharing account that I set up for limited access. You can use the Guest account for this purpose, but I’d rather control it more directly by name. See also how to set up safe user accounts on a shared Mac.

Remote Login, Management and Apple Events

Apple offers powerful but somewhat obscure ways to access, manage, and use a Mac remotely:

• Remote Login lets you connect via the SSH (secure shell) protocol, commonly used in Unix/Linux, as well as access file sharing with SFTP (Secure File Transfer Protocol), which is really “FTP using SSH.” It’s rarely needed except for particular purposes, such as if you need remote command-line access. If you do enable it, keep “Allow full disk access for remote users” turned off.
• Remote Management connects with Apple Remote Desktop (ARD), a corporate and academic tool. If you’re not using ARD, don’t enable this service.
• Remote Apple Events or Remote Application Scripting is a tweaky thing that lets an AppleScript running on one Mac send control events to another. It’s rare you need this, and you will know if so.

Reaching your Mac from outside

If you want to have access to Macs and other devices that are behind NATs or other network protections, I highly recommend Tailscale. The company’s core product lets you create an account, install software on nearly any device, and then have those devices create VPN tunnels to a pooled central network space. This is a profoundly straightforward way to use File Sharing and Screen Sharing when away from your home network. You can install Tailscale on an Apple TV and then set it up to provide access to the entire network it’s on, too!

Tailscale offers a non-commercial account that fits most people’s personal needs.

Apple keeps upping the ante on how it makes sure that macOS’s core files—all the bits and pieces in the operating system that allows apps to run—aren’t messed about with.

What if Apple could prevent system files from being modified at all by placing them on a read-only disk, effectively blocking changes to those files at nearly the lowest level?

That was a hard task, given that system files and user data files co-existed on the same startup volume. But, hmm, wait a tick! What if you could split system files into one volume and data files into another, but have them appear seamless as a single “drive” in macOS? The system volume would be read-only; the data volume could be read/write, but would also have all the sandboxing protections already in place.

That’s exactly what Apple did starting with Catalina. Apple’s modern filesystem, APFS (Apple File System), among other improvements, added the concept of breaking a drive into containers instead of partitions. (Partitions can still be used, but there’s no advantage.)

In the long-used previous filesystem, Mac OS Extended (sometimes called HFS+), a drive was broken into partitions, and each partition could be mounted as a volume. One volume was much like another.

In APFS, a drive is broken into containers, which are like partitions in occupying a preset portion of the disk’s full capacity. Each container can contain one or more volumes, and each volume can have a role. A role defines the kind of data stored on it. Roles include data (for regular mountable data volumes), system (for system files starting in Catalina), and backup (for Time Machine backups starting in Big Sur).

Roles also include obscure and/or invisible system requirements, too: Preboot, Recovery, and VM (virtual memory).

Catalina’s system role also added another variation on containers and volumes, called a volume group. A volume group is two or more interrelated volumes that present as a single entity to the Finder and user. Behind the scenes, however, multiple volumes are managed by the system.

The reason for this was to take a previous system integrity concept to yet another level: all system files are on one volume in the startup volume group; all user data is on another volume.

Big Sur went even further. It doesn’t even mount the system volume. Instead, it uses a “snapshot” feature of APFS that allows the filesystem to capture a particular point in time. Starting in Big Sur, a snapshot of the system volume is loaded that can’t be changed, because it has no writable components—it’s like looking at a picture. In Disk Utility the system is marked as an APFS Startup Snapshot with a unique name, while the main system volume is dimmed.

On top of that, each file on a Big Sur or later system volume has a separate cryptographically generated hash that’s stored in the volume’s metadata. Whenever a file is read from the system volume, that same crypto operation is performed, and the resulting hash of the loaded file checked against the one that’s stored—any modification, however unlikely it could be to occur at all, would be immediately spotted. That’s why this approach is called a Signed System Volume. The metadata and other volume attributes are hashed and collected in something called the seal, which is verified at boot time. If the seal can’t be verified, you’re prompted to reinstall macOS.

Monterey kept the same structure, but added the capability for Apple silicon Macs to install multiple versions of macOS on a single drive. With Big Sur, an Apple silicon Mac could only have a single system because of some low-level decisions about how the Mac decided whether a system could validly start up. With Monterey, you can create additional partitions and install unique copies of macOS into each partition. (From Ventura onward, Apple doesn’t seem to have made substantial changes—or at least I couldn’t find any to report on.)

Only some people need to have multiple versions of macOS on a bootable drive, such as those who need to keep older versions of macOS running for testing or compatibility.

With Apple silicon Macs, Apple also has what’s called hardware-based memory protection. Hackers often use a system or app exploit to overflow an area reserved for pure data into an area that contains executable program code. This lets them insert malicious code that is run in place of legitimate code. Memory protection in Apple silicon processors marks memory areas as either full of executable code or full of data, but not both. In that scenario, a hacker cannot write malicious code into an area that can be executed; nor can they execute code that’s in a place only inert data is stored. An app can change the state of memory areas explicitly, and that’s a place that hackers will certainly aim for. But it’s much higher-hanging fruit.

System integrity and locked apps

Because the system volume is immutable, Apple can place only certain of its apps on that volume: ones that don’t require regular updates. These apps can be refreshed as part of a system update, as a Catalina or later system update has the rights to make changes to the system volume, including those apps.

The list of system-locked apps is reasonably long. You can find them in /System/Applications. A few examples among many are App Store, FaceTime, Mail, Photos, Preview, and Siri.

System volume apps appear within the Applications folder intermingled with Data volume apps. This is part of the seamless integration of volumes in a volume group used for macOS. You can check on the system/Data location of any app by selecting it and choosing File > Get Info. The path shows drive name > System > Applications for system volume apps, and drive name > Applications for ones installed on the Data volume.

Interestingly, Safari is not on the system volume. Apparently, it’s updated regularly enough and sometimes with significant fixes that Apple has kept it out of that fixed side of things.

In a similar but distinct bit of processor-based protection, after macOS loads on a Mac with Apple silicon, the memory its central components occupy—its kernel—is locked using “kernel integrity protection,” so they cannot be modified while macOS is running.

Do you feel safe now? You should to the extent that it’s feasible that Apple has taken every modern and many inventive measures to render the system immutable.

How to restart in recovery mode

Apple installs a special recovery volume as part of a macOS installation that you can restart from to perform actions on your Mac’s system without macOS itself running. This volume is invisible to you in your normal use of macOS.

On an Apple silicon Mac, choose Apple menu > Shut Down. When your Mac has powered down, hold down the power button; you first see a message that says “Continue holding for startup options.” Keep holding until you see the next prompt, which says “Loading startup options.” Click Options, choose an account, click Next, enter its password, and click Continue.

Apple changed the name for this form of boot a few years ago. Recovery mode generically means a special way of booting any Apple device to repair or reinstall its operating system.

When you restart into recovery mode, Apple’s Platform Security Guide says you are booting into recoveryOS. The app that appears first on an Intel Mac or after clicking Options on an Apple silicon Mac is labeled Recovery on the system menu.

Furthermore, if you choose the Utilities menu, you can launch a special Recovery Assistant! The main screen there is labeled not Recovery Assistant, but simply Recovery—see below. If you lock your Mac via Find My and then unlock it, Recovery Assistant launches on restart, and while the app menu reads “Recovery Assistant,” the main title on the screen is “macOS Recovery.”

Startup protections

This wasn’t enough? Really? Really. There’s more. Apple has several methods of further preventing your Mac from being started up in a way you don’t want and to prevent after startup in ways you don’t want.

Share a disk

As part of enhanced protections on Apple silicon Macs, Apple eliminated a simpler option long used on Intel Macs. Follow these steps to share a volume, making it appear as a networked volume on the other Mac:

1. Restart in recovery mode. (See How to Restart in Recovery Mode.)
2. Choose Utilities > Share Disk.
3. Select the disk to share, and click Start Sharing.
4. If prompted, choose an account, enter its password, click Unlock, and click Start Sharing.
5. Connect your Mac to another one via a USB data cable (with Type-A or USB-C plugs on either or both ends) or a Thunderbolt 3 or 4 cable.
6. On the other Mac in the Finder, click the Network link in the sidebar to view the shared Mac’s volume.
7. Click the Mac in the main window, click Connect As in the upper-right corner, select Guest as the user, and click Connect.

You can now transfer files between the two computers. When you’re finished, eject the mounted Mac volume by selecting it and dragging it to the Eject icon in the Dock or pressing ⌘-E.

Startup Security Utility

Apple took a harder line on startup security policy when they introduced Apple silicon Macs than they previously had with Intel models. macOS offers Full Security, which locks your Mac down to either its current version of macOS or any version Apple currently supports—typically a limited set. Reduced Security lets you run older versions of macOS that Apple previously approved to run on your hardware.

Almost unrelated, the “Allow accessories to connect” option lets you set a boot policy to restrict connections. For options, see the Mac settings worth changing.

Reduced Security lets you select or deselect the option to install signed legacy (or outdated) kernel extensions; and for remote management, which is used in schools and businesses, to control these kernel extensions and software updates. Some organizations may need specific older extensions for security software and may also want to delay automatic software updates to avoid breaking them.

People at a heightened level of risk could trust Reduced Security with just user management enabled for kernel extensions, since those extensions must be signed by “identified developers” (via Apple’s process) and require user steps to install, as described in Manage System Extensions.

If you don’t have Reduced Security enabled and try to install a system extension, the app installation explains what happened.

Proceed to click Open System Settings, and you see a note in System Settings > Privacy & Security.

Click Enable System Extensions and you see a dialog that provides a shortcut and offers abbreviated and somewhat misleading steps on enabling Reduced Security. If you want to proceed, click Shut Down in that dialog and then follow from step 2, below.

Starting from scratch, follow these steps:

1. Restart in recovery mode. (See How to Restart in Recovery Mode.)
2. Choose Utilities > Startup Security Utility.

Note: Apple requires an internet connection when you change the security policy, and you can hit a snag if you follow step 2 too quickly. If you see the error “An internet connection is required to change security policy,” quit the app (labeled Startup Disk when launched), wait for the Wi-Fi icon to show a connection in the menu bar, and launch the utility again. (An Ethernet connection doesn’t work.)

1. The app launches. Click Unlock to mount the disk, which will be encrypted.
2. Select an administrator user, enter the account’s password, and click Unlock.
3. Select the drive if it’s not already selected.
4. Click Security Policy.
5. Select the level of security you want to apply and click OK.
6. When prompted, select an administrator user, enter the account password, and click OK.
7. “Applying security policy” appears. It may take several to tens of seconds to complete.
8. Quit the utility and choose Apple menu > Restart.

Tip: Reduced Security disables Apple Pay on a Mac with Touch ID capability. See how to set up Touch ID and Face ID.

Permissive Security (not shown in this section) is an additional option that can’t be selected without disabling an even lower-level feature: System Integrity Protection. The Startup Security Utility then adds Permissive Security as an additional radio button. However, it’s not “no security,” but rather “experimental security”: it allows a very particular kind of installation, in which researchers and other niche users create custom kernels—the heart of the operating system—outside of the Apple-signed ecosystem of macOS versions compatible with Apple silicon Macs. It is exceedingly unlikely you would ever need it.

Recovery Assistant

Apple added Recovery Assistant in Tahoe to deal with situations in which macOS couldn’t start up correctly, and some sort of repair was required that needed the startup volume unmounted. If you encounter this, you will see a Recovery screen that explains the issue. You can also launch Recovery Assistant from macOS Recovery by choosing it from the Utilities menu.

The process works like this:

1. At the Recovery screen, click Continue.
2. Apple asks if you are willing to send them diagnostic data, and warns you about privacy issues. Click Don’t Send Data to Apple or Send Data to Apple.
3. Click Start, and the assistant tries to resolve the problem.
4. The assistant reports one of three states:

1.1. Your Mac was recovered successfully. Click Restart Mac.

1.1. No known issues were found. Click Restart Mac.

1.1. Your Mac could not be recovered. Oh, boy, it’s time to look into backups, consult an independent Apple expert or the Genius Bar at an Apple Store, or call Apple Support.

If you open System Settings > General > Sharing, you’ll notice many resources your Mac can share with other devices on your local network—and, in some cases, beyond.

You can share your screen, files, printers, and internet connection, for example, and you can also enable various types of remote access to your Mac.

All these features can be handy, especially in that they enable multiple Macs in your home or office to talk to each other. You can copy a file from a Mac in the other room, or view what’s on the screen of the Mac upstairs when you’re downstairs.

However, Apple pairs easy local access with easy remote access: if your Mac is reachable from the internet, some services you share locally can be available remotely. Some may require a password, but that could be easily guessable if you haven’t been thinking about internet-based attacks. And just having certain services active, like Remote Login, could allow remote attacks if an exploit turned up in Apple’s system software, before such exploits were known and patched.

This isn’t speculative. Apple regularly patches exploits and bugs in low-level software, like sshd, the secure shell daemon, the system software that handles remote Terminal-style sessions if you have Remote Login enabled. Those flaws are usually fixed before anything terrible is unleashed in the wild, however.

How exposed you are comes first; which sharing services are safe to turn on follows from it.

What’s your risk of internet intrusion?

The most likely determinant of risk to attacks or intrusion via the internet is how directly reachable your Mac is. While the vast majority of routers have a public IP address, very few computers on local networks do. Almost always, your router receives traffic and then re-addresses it to your Mac.

A public IP address is by definition routable or reachable from the rest of the internet. The network of which it’s a part appears in routing tables traded among the devices that exchange data across this mighty global beast.

However, IP addresses can also be static or dynamic. A static address is assigned to a device and doesn’t change over time without typically manual involvement in changing settings. A dynamic address can change at will, and it’s automatic.

Private network addresses can be either static or dynamic; most of us have networks configured to assign the next available address when a computer, phone, or other device requests one. Some routers can be configured to assign static private addresses to specific devices.

If you’re not running a business network or paying extra, your broadband router typically has a dynamic public address. The ISP may rotate the address every once in a while, or if there’s a service outage or you restart the router, a new address is assigned.

The dynamic nature of these public addresses assigned to routers doesn’t aid in security in any fashion, because they’re still public. You might not be trackable by public IP, but your router is still reachable.

A public IP address is simply one that’s uniquely assigned across the internet, and can be reached from anywhere else on the internet. Your router almost certainly has a public IP address, because that’s how it interacts with the rest of the world on your behalf.

It’s a very low bar for any interested party to scan all the originally defined set of public addresses on the internet—yes, as many as hundreds of millions of them—to find open access for services that the scanner might know potential security exploits for, sometimes far out of date…but there are a lot of old, unpatched machines on the internet. Thus, with a public address, there’s no obscurity possible.

That “originally defined” part uses an addressing scheme called IPv4; a newer standard, IPv6, is more resistant to bulk scanning. See the IPv6 section below.

Most homes and businesses rely on private IP addresses for everything but their broadband connection’s router, however. These reserved ranges are only used on local networks, are managed by a router (using something called Network Address Translation or NAT), and typically assigned out automatically with DHCP (less well known as Dynamic Host Configuration Protocol).

Private addresses, which are usually in the form 10.0.1.x or 192.168.1.x, can be reached without any special effort by other devices on the same network of privately assigned addresses. However, when a device on your network makes a request outside its private network range, like viewing a webpage, the router rewrites the request to use the router’s public IP address, and sends it out; the router receives a reply and then forwards it to the locally connected device that requested it.

If you were to have a public IP address on your Mac—not just on your router—it would be like standing on a street corner instead of being inside a house with a locked door and a mail slot. Anyone can come up to and start talking to you or assault and rob you! (Unlike in the world of atoms, where violent crime is minimal and has drastically fallen in recent years, an internet mugger can attack targets wherever they are and a huge number simultaneously.)

The same effect happens if you configure your router to connect its public address with specific services on one or more devices on the local network. For instance, some people want to have a globally available web server or file server, and it’s not terribly hard to expose the correct connection. That makes that web server or file server just as exposed as if the Mac had a public IP address—but only those services, not everything else shared on the Mac.

NAT isn’t designed as a security measure: it’s just a spreadsheet the router maintains to wire requests and replies together. It’s incidentally a general security measure because it makes potentially exploitable open doors on your devices nearly impossible to reach without being specifically targeted, particularly with a router that can be subverted.

You may already be sure you don’t have a public IP address on your Mac: your ISP charges extra for public addresses you use beyond the one attached to your router, and you know you’re not paying for it; your ISP doesn’t offer public addresses; you specifically made sure you weren’t getting one from your ISP; or you’ve configured your router and you know exactly how you set things up.

If you’re not sure, walk through this list:

• Check your Mac’s address: You can examine the IP address assigned for your active network interfaces. Go to System Settings > Network and select each of your active interfaces in the main pane:

— Ethernet and most interfaces show the address in the main pane under or next to “IP Address.”

— For a Wi-Fi interface, click Details to see the address next to IP Address.

• Check your router: Connect using the administrative controls for your router, log in, and look at where your router shows its WAN (Wide Area Network), Modem, or Internet IP address, and how network addresses are handed out under DHCP, LAN (Local Area Network), or similar labels. The WAN side is almost always a public IP; the LAN/DHCP part almost always private.

If the IP address of your Mac or LAN/DHCP addresses on the router is in any of these ranges, it’s a private one, where x is any number from 0 to 255: 10.x.x.x, 172.16.x.x to 172.31.x.x, or 192.168.x.x. Otherwise, it’s almost assuredly public.

Type in “what’s my IP” at Google and it will tell you what it thinks your IP address is, and offer a list of sites that do the same (sometimes with more detail). However, if you’re in a DHCP/NAT scenario with private addresses, which is what nearly all home users and most business users are, the address shown on the website is the one associated with either your broadband modem or a further upstream router run by the ISP—not your Mac.

IPv6 adds a wrinkle

Technically, the IP address style x.y.z.a, like 192.168.0.1 or 36.44.0.6, is IPv4 (version four), which has been in use for decades. For nearly 30 years, internet mavens have been trying to migrate the networks that comprise the internet to IPv6, which has a lot of advantages. This includes more addresses that can be assigned, the so-called address space. Where IPv4 has billions of potentially usable addresses, IPv6 has hundreds of undecillions.

However, the trouble has been all the inertia of having billions of devices and hundreds of millions of routers and pieces of plumbing designed around IPv4. While the internet has largely retooled—after nearly 30 years—so that IPv6 works as well as IPv4, nobody wants to end the widespread use of IPv4 in residential networks and many company networks, because of private addressing. IPv6 was designed before NAT became broadly used and thought of as a quasi-security measure to prevent ingress to ISP and corporate networks.

For most of us, that means until private addressing can work in a similar way within IPv6, our ISPs have assigned our routers a public IPv4 address, and have chosen one of several paths with IPv6:

• Ignored/disabled: Your ISP may simply not pass IPv6 traffic, and it’s not an issue for security.
• Allow, but disabled by default: You can use IPv6, but you have to configure your router to allow it. My ISP requires a special router for its fiber-optic service, and has detailed instructions for enabling IPv6. Since I have no specific need of it, I’ve left it disabled to reduce my exposure on the public internet.
• Turned on by default: You may have an ISP, like my editor Dave Johnson, who enables IPv6 by default through its system, and your router and devices all automatically get IPv6 addresses, just as they do IPv4 ones.

If you’re in that last situation, you may have publicly reachable IPv6 addresses—it can be hard to tell, because some ISPs use techniques that are similar to NAT. If the IPv6 addresses are truly publicly accessible, they expose the same general risk of remote attack as public IPv4 addresses and should follow the advice in the next section.

But some analysis suggests it’s not as bad as it might seem. The Internet Society has an interesting FAQ about IPv6 and security, and notes that attackers could find “infrastructure nodes” reasonably easily—the routers that form the backbone of the internet and connect ISPs, streaming services, data centers, and so forth all together.

But, the document explains, “It is generally unfeasible though to address scan a network for client devices, since their addresses are randomized over a very large address space.” Simplified, it means that it’s not a needle in a haystack, but 10,000 needles in a Nebraska-sized area of haystacks. (With IPv4, it’s a cubic foot of hay and you can see all the needles inside.)

If you’re truly concerned, you can disable IPv6, as it isn’t a mandatory or critical part of using the internet. Here are three possibilities, the first two of which will affect your entire network:

• Disable IPv6 at the router: Find the manual for your router and follow instructions to disable IPv6. It may be as simple as selecting a radio button.
• Ask your ISP to disable IPv6: Contact your ISP and ask them to disable IPv6 on your account or router if they control your router or have the ability to do this at a higher network level.
• Disable IPv6 on your Mac: If you can’t or don’t want to disable IPv6 network-wide, disable IPv6 on your Mac. Go to System Settings > Network, select an interface, and click Details. Select TCP/IP and choose Link-Local Only from the Configure IPv6 pop-up menu. Click OK. This leaves other devices still reachable via IPv6, but at least locks down your Mac.

You can’t disable IPv6 on an iPhone or iPad.