If you have more than one computer, you will want to set up a little network. Buy a router, a box that connects downstream of your cable or DSL modem, and that lets you connect several computers to it. For example, the Linksys BEFSR41, about $50, or a wireless/wired router like the Apple AirPort Extreme ($100-180). (An Apple Time Capsule does the same thing and also has a hard drive in it for backup.) Using a router allows you to share the ISP connection with multiple computers, and also allows computers on your home network to communicate and share files.
This diagram shows a network with several types of computer. The local computers are connected by Ethernet cables; guest computers connect by wireless. Wired connections are faster, less subject to interference, and less vulnerable to security attack.
For advanced information on home networking, read Glenn Fleishman's "Take Control of Your 802.11n AirPort Network, Second Edition".
(Nov 2016) Apple has announced that it will abandon development on wireless routers. So no new Airport Extreme or Time Capsules. I guess this means no more security updates? This is very disappointing.
Your router should be configured as a DHCP Server. That is, it makes up local Internet Protocol addresses for each device on your local network, and sends all devices' outgoing packets out over the the external Internet Protocol address. When a response packet comes back, the router matches up the packet sender with that of a previous request, and forwards the response to the local machine that is expecting it. Unsolicited messages are dropped, protecting your local computers from a lot of attacks.
Some routers, like the Apple AirPort, allow you to connect a USB disk drive, and use it for backup and file sharing from computers on your internal network. I used this feature and it worked OK. Apple now says that this feature is only supported on certain AirPort models; the Time Capsule is an AirPort router with a built-in hard disk.
(By the way, do not configure your router to hide the network name (SSID). This actually decreases security. Make up a random string for the name.)
There have been recent reports of security problems with home routers. Some routers have "back doors" built into them; others can be hijacked if you visit a malicious web page.
If you are really interested in securing your network, read http://lifehacker.com/how-to-tap-your-network-and-see-everything-that-happens-1649292940
Your AirPort or Time Capsule has an administrator name and a password that lets you configure it. Pick a long password.. you won't be giving it often. Uncheck "allow setup over WAN." I don't enable "Back To My Mac." The "Internet" tab specifies how your router connects to the rest of the world. This depends on your Internet connection. I set mine to "DHCP." The "wireless" tab specifies how your router supports WiFi. Choose "WPA2 Personal" for encryption, and pick a random network name and a strong password. You can also create a "guest network" for visitors to your premises: they won't be able to see your computers or printers. The "Network" tab says what kind of network your router provides to WiFi devices. Mine is set to "DHCP and NAT."
Your router may have an option to enable IPV6. (This is a more advanced network address format, that will become widespread in the future.) My Time Capsule has three possible settings: Link-local, Node, and Tunnel. If you select, then your computers inside your home can talk to each other using IPV6, but nobody from outside can use IPV6 to access them. This is good: it is how mine is set up. You will be able to access sites on the Internet, no matter what kind of address they use. If you select or , and don't block incoming IPV6 elsewhere, then your computers can be seen and attacked from distant machines.
If you need to connect more computers or devices to your network than there are ports on your router, use an Ethernet hub. Modern Macintoshes can send data very fast, at Gigabit Ethernet speeds, if both ends of the channel can handle this speed. If you buy a router or hub, make sure it supports Gigabit, or 1000base-T, speed. When you are buying Ethernet cables, choose "cat 6" or "cat 6A" cables to make sure you get full speed.
Printers can be connected
To use a printer on your home network and print to it from all your local computers (1 or 2 above), it needs a print server. I have a Brother HL-4150CDN printer, and the "N" means it has a built-in print server and can be connected to a network.
If your router supports WiFi, then you don't need wires between the computers and the network. Visitors can temporarily use your Internet connection too. Be aware of the security implications: configure your router to require a name and password, and use WPA-2 encryption.
Understand that WiFi speeds are routinely overstated. WiFi is also subject to interference from other radio devices.
Once your network is set up, you can connect multiple computers, phones, and tablets to the network. To connect a Mac, do this:
Other users on your network can use files on your Mac if you enable file sharing. Selectto open the Sharing control panel. This is where you choose a network name for your computer. You can also enable many kinds of sharing: allows other computers to access files on your machine. allows computers to see your screen and move your mouse. You only have to enable sharing once, and it will remain enabled till you turn it off, even if you restart the Mac. Do not enable features unless you understand their security implications.
In order to access files on another Mac, call it Al, on your network, enable sharing on Fred as described above.
Open a Finder window. Navigate it to the folder containing your local files, or where you want to put files you copy.
Open a second Finder window with command-N: you will use it to view a folder on the remote computer.
In the left sidebar you will see a heading
On the first visit, the Finder will ask you for a password. Give the name and password of the computer's owner and check .
If you don't see the remote computer you want to share files with, type command-K in the finder, and enter afp://machinename.local/.
Now you should have a list of folders you can open on
the remote computer.
Double click on the folder you want and navigate around as usual. When you drag file or folder icons between the two finder windows, they will be copied from one computer to the other.
When you are done sharing files between computers you can click the little eject button (⏏) next to the remote computer's name in the Finder side bar, to disconnect. If you don't do this, you will get a warning dialogue when the remote computer reboots, and files could be damaged.
If you clickwhen you open up a remote Mac in the Finder, and if you have allowed screen sharing in its , then the Screen Sharing application will launch on your computer, showing the screen of the remote computer. You can drive the cursor and send mouse clicks to the remote computer, and type into its applications. This can be very valuable for configuring remote computers, recovering from bad video settings, and assisting other Mac users.
Screen sharing like this can be done across iChat to view and control a remote Mac anywhere, which can be very useful for remote support of friends and relatives.
You can do some file sharing from your Mac's finder to Windows computers on your home network. I don't use this much, and sometimes it is complicated, so I won't go into it here. A simpler alternative is to install Dropbox on both Mac and Windows.
(For advanced users.)
I often use scp, rsync, and ssh to access Linux and Unix computers on my network (and elsewhere). These are commands that come with every Mac (install the developer tools). These commands will also work for Mac to Mac (since Mac OS X is based on FreeBSD Unix). (I tried screen sharing with a Fedora Linux computer, and it popped up a dialog box asking permission, but then nothing happened.. need to debug this someday.)
You can type man ssh in a Terminal window to get standard Unix help, useful to Unix programmers.
These commands will keep asking for the password of an account on the remote system, which gets tedious. You can arrange things so that you only have to give a password once.
For example, I do this command to sync my home directory from my laptop to another computer (all on one line):
rsync -avzu --blocking-io -e "ssh" --exclude .Trash --exclude Cache/ /Users/thvv $user@$host:/Users
where $user and $host are shell variables set by the export command.