How do I figure out my IP address on a Mac?

If you're running Dynamic Host Configuration Protocol (DHCP) then you're right, you'll get a new IP address (possibly recycled) each time you connect to the Internet. Well, you actually get what's called a "lease", so you only get a new address when your lease expires. Typically DHCP servers are configured to give 24 hour leases, so it's not quite as much a moving target.

The easiest way to identify your IP address is to pop open the Terminal (go to Applications -> Utilities -> Terminal) and type in the interface configuration (ifconfig) command:

$ ifconfig
lo0: flags=8049 mtu 16384
        inet6 ::1 prefixlen 128 
        inet6 fe80::1 prefixlen 64 scopeid 0x1 
        inet 127.0.0.1 netmask 0xff000000 
gif0: flags=8010 mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863 mtu 1500
        ether 00:30:65:3d:e8:10 
        media: autoselect ()
        supported media: none autoselect 10baseT/UTP ...
en1: flags=8863 mtu 1500
        inet6 fe80::230:65ff:fe03:25bc prefixlen 64 scopeid 0x5 
        inet 10.0.0.104 netmask 0xffffff00 broadcast 10.0.0.255
        ether 00:30:65:03:25:bc 
        media: autoselect status: active
        supported media: autoselect
fw0: flags=8822 mtu 2030
        lladdr 00:30:65:ff:fe:3d:e8:10 
        media: autoselect  status: inactive
        supported media: autoselect 

The number you want to identify is immediately after the "inet" field. Rather than just scan this visually, however, let's use some Unix commands to extract the data we want.

The first command we'll use is grep, a simple pattern matching filter. We'll make what Unix geeks call a "pipe" by separating the two commands with the "|" symbol, which causes the output of the first command to be fed to the second command as its input. Put them together:

$ ifconfig | grep "inet"
        inet6 ::1 prefixlen 128 
        inet6 fe80::1 prefixlen 64 scopeid 0x1 
        inet 127.0.0.1 netmask 0xff000000 
        inet6 fe80::230:65ff:fe03:25bc prefixlen 64 scopeid 0x5 
        inet 10.0.0.104 netmask 0xffffff00 broadcast 10.0.0.255

A lot better already! Now, let's narrow it down to just the "inet" fields, not the "inet6" (which is actually IPv6, but that's beyond the scope of this discussion) by adding a space to the pattern:

$ ifconfig | grep "inet "
        inet 127.0.0.1 netmask 0xff000000 
        inet 10.0.0.104 netmask 0xffffff00 broadcast 10.0.0.255

Almost done. The second line has the real IP information for my computer because the IP address 127.0.0.1 is special, it's called your "loopback" address and always refers to your own computer, regardless of if you are running a Mac, PC, Linux or any other sort of machine. Just part of the definition of the underlying TCP/IP protocol.

To get rid of that spurious match, I'll use grep again, but this time I'll include the '-v' flag, which reverses the logic of the search (that is, it'll match all lines that do not match the specified pattern):

$ ifconfig | grep "inet " | grep -v 127.0.0.1
        inet 10.0.0.104 netmask 0xffffff00 broadcast 10.0.0.255

That's short and sweet. One more step, just a little one, to remove the stuff we don't really care about in the output, okay? For this, I'm going to use cut, a great command line utility, to show me just the second field in the line, using spaces as a delimiter:

$ ifconfig | grep "inet " | grep -v 127.0.0.1 | cut -d\  -f2
10.0.0.104

Perfect! Now, final step, save this as a Bash alias by typing in the following (exactly):

$ echo alias myip="ifconfig | grep 'inet ' | grep -v 127.0.0.1 | 
   cut -d\   -f2" >> ~/.bashrc

Exit your Terminal window and launch a new one, and from this time forward you'll always be able to simply type myip to find out what your current IP address is, all within the terminal!

Hope this helps you out!