Dave, as far as I know, I get a new IP address every time I connect to the Internet with my Apple PowerBook. How do I figure out what address I’ve been assigned?
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.
There are some utilities and apps you can use, and for that matter you can also go to “System Preferences…” off the Apple menu and look at your “Network” panel, but let’s stick with “ifconfig” because it’s a bit more interesting to use the command line…
$ 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!