Finding Your External IP Address

For various reasons you might want to find your public IP address, that is, the 32-bit number, expressed in pseudo-decimal form as something like 72.14.204.99, that tells the world how to reach your computer.

For most businesses and government offices this is trivial: your computer is assigned a static IP address, one that never changes with time. For home users, though, it's a little more complex.

Start with the IP address itself. The largest possible IP address is 255.255.255.255. That means that you can have no more than 4,294,967,296 unique IP addresses, and some of those are reserved for local use, as we'll see. We're coming up on 7 billion people in the world. Not everyone has a computer (yet, and remember iPhones, iPads, Kindle, and all 'Droids count), but many people have more than one (see iPhones, iPads, …). So there aren't enough IP addresses to go around. Eventually, everyone will switch over to the IPv6 address scheme, which will give us 340,282,366,920,938,463,463,374,607,431,768,211,456 (2¹²⁸, or 3.4×10³⁸) possible addresses, which is probably enough for every non-hydrogen nucleus in the observable universe to have its own address.

But we're not there yet, so something has to be done to shrink the address space. Your local ISP solves this problem by 1) charging you extra to get a static IP address; 2) giving you only one IP address per gateway (your cable, FIOS, DSL, or dial-up modem, or some other router or hub that connects you to the internet); and 3) taking your IP address away from you when you're not using it.

That last one is called dynamic IP addressing, and it's part of what we're going to talk about today. If you have a dynamic IP address, then when you turn your gateway connection (usually some type of modem or router) on, your ISP assigns you an IP address from its bank of addresses. As long as your connection is turned on, you'll have an IP address, but your ISP can change it at any time. It will almost certainly change it if you turn off your connection and turn it on again. So if you know your IP address right now, there's no reason to expect it will be the same tomorrow, or even five minutes from now — though to be fair, Verizon FIOS has kept our house on the same IP address for weeks.

And there is worse to come: Suppose your ISP has given you the address 244.117.16.25. The computer you're reading this on doesn't have a clue as to what that address is. Go back to 2), above. Your house has only one IP address, and it belongs to your gateway. Every other computer in the house belongs to your home network. When you want to download your email, your computer politely requests the gateway to go fetch it:

Please, sir, may I have my email?

Oh, all right. Just this once. But don't tell any of the other computers what we're doing.

Thank you, sir! I'll be eternally grateful

You'd better be, or no Internet for you.

Let's see how this works. ping is a command that will poke a stick at a computer and get information about it. Let's try it with a well known site:

ping google.com
PING google.com (72.14.204.99) 56(84) bytes of data.
64 bytes from iad04s01-in-f99.1e100.net (72.14.204.99): icmp_seq=1 ttl=53 time=21.8 ms
64 bytes from iad04s01-in-f99.1e100.net (72.14.204.99): icmp_seq=2 ttl=53 time=20.2 ms
64 bytes from iad04s01-in-f99.1e100.net (72.14.204.99): icmp_seq=3 ttl=53 time=23.4 ms
^C

(you'll need to hit ctrl-C to stop this)

So we see that google.com has an IP address of 72.14.204.99 — that's actually one of many, but don't worry about that. Now let's try it with a local machine:

ping george
PING george.home (192.168.1.2) 56(84) bytes of data.
64 bytes from george.home (192.168.1.2): icmp_seq=1 ttl=64 time=9.77 ms
64 bytes from george.home (192.168.1.2): icmp_seq=2 ttl=64 time=2.70 ms
64 bytes from george.home (192.168.1.2): icmp_seq=3 ttl=64 time=0.961 ms
^C

So far has the home network is concerned, george lives at 192.168.1.2, which is one of those reserved addresses we mentioned above. The range 192.168.0.0 to 192.168.255.255 is reserved for internal networks. As we read this, there are no doubt hundreds of thousands of computers with the local IP address of 192.168.1.2. Every one of them, however, has a different public IP address.

Great. But sometimes I need to know my external IP address, and it's apparent that Hal, here, hasn't got a clue as to what it is. How can I find it?

The easiest way is to go to another website. Every website you connect with gets your public IP address automatically, unless you use some kind of anonymous service. Some websites are set up to tell you what your IP address is. One example is What Is My IP?. They even provide a way to get your IP address with a simple one-line terminal command:

wget www.whatismyip.com/automation/n09230945.asp -O - -q ; echo

If you read your Gmail via Thunderbird, Evolution, Outlook, etc., and keep your web client on all day, gmail will tell you the address of your computer. Just log on to your gmail account and look for the line that says Last account activity: …. The IP address there is the last computer to download email. If you're using more than one computer from different locations, say you left your office machine on over the weekend and now you're using a home computer, click on the Details link and you'll find all of the activity for this account.

There's a company DynDNS, which will not only find your IP address, but give you a domain name that constantly updates to your current numerical address. Of course, they do this by having your computer constantly tell them the public IP address.

And, of course, your gateway knows its public IP address. Just log onto it through your browser (something like http://192.168.0.1, http://192.168.1.1, or http://192.168.100.1, depending on your system), and your IP address will be up there someplace.

But all of these systems have flaws: Say you need your IP address in a script. You can get it via your browser from your gmail or your gateway machine, but it's hard to parse a browser page inside a script. The What's My IP? one-liner works in a script, but what if their site goes down, or, worse, they go out of business? Same for DynDNS. We want a solution that works any time your gateway is on and your computer has an internet connect, and we want to get it from the command line.

Any other way just ain't elegant.

If you search the Internet you find surprisingly few ways (i.e., none) to do this. Partly I think it's because there isn't a consistent way to find your gateway from your current machine. For example, in the days when we used Comcast, we had a cable modem as the gateway. On the house side of the modem we connected a Linksys router, which was the hub of our home network. All the computers in the house were connected through the router to the modem — a two step-process. Now we have Verizon FIOS, with a Actiontec MI424WR router that serves as gateway/modem and router — a one-step process.

By stumbling around, and with a little help from nixCraft, I was able to come up with a solution that works for me. It's a bash script. Let me put it here (finally, I hear you say), and we'll dissect it below:

#! /bin/bash

# Determine the public IP address for your DHCP system by bootstrapping

#  calls to your gateway/router

# Requires "route," available in Ubuntu package net-tools, and
# nslookup, available in Ubuntu package dnstools

# Disclaimers:
# 1) This works with Verizon FIOS using the Actiontec MI424WR Router
#    If you have another system YMMV.
#
# 2) And, as you'll no doubt note, this is set up for a Debian system,
#    specifically Ubuntu 10.04 .  Package names and installation
#    procedures for other systems WILL vary.
#

###
# Start Here:
###

#
# Get Router internal Gateway IP
# Information found on
# http://www.cyberciti.biz/faq/how-to-find-gateway-ip-address/
# If route is not installed, run
# sudo apt-get install net-tools
# and make sure /sbin is in your $PATH
# Note "U" = up, and "G" = gateway

internalip=`route -n | grep UG | awk '{print $2}'`

echo Your Gateway\'s Internal IP Address is: $internalip

# Now we need to get the name of the router.  If you don't
#  have nslookup installed, run
# sudo apt-get install dnsutils

echo -n "Your Gateways's name is: "

routername=`nslookup $internalip | grep "name =" | awk '{print $4}'`

# On the Actiontec, at least, the name has an annoying "." at the end.
# Remove it:

routername=${routername%\.}

echo $routername

echo -n "Your Public IP Address is: "

nslookup $routername | grep Address | tail -1 | awk '{print $2}'

OK, what's going on? First of all script only works if you have the route and nslookup commands installed on your system. If they aren't there, in Ubuntu, and presumably and Debian system, you can install them using

sudo apt-get install net-tools dnsutils

The first call, route -n, will look at the Kernel's IP routing table:

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     0.0.0.0         255.255.255.0   U     1      0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 eth0
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth0

which on this machine tells us that we have several local connections. The U under Flags means that the connection is up, and G means it's the gateway. From this we see that the numerical address of the gateway is 192.168.1.1. Let's see what nslookup can tell us about that. Note that the script has to parse all of the output to get this far.

$ nslookup 192.168.1.1
Server:  192.168.1.1
Address: 192.168.1.1#53

1.1.168.192.in-addr.arpa name = Wireless_Broadband_Router.home.

Here's something new, our gateway's got a name. Wireless_Broadband_Router.home. Again, the script has to do a little work to tease that name out of nslookup's output, but we'll just do it by hand here:

$ nslookup Wireless_Broadband_Router.home
Server:  192.168.1.1
Address: 192.168.1.1#53

Name: Wireless_Broadband_Router.home
Address: 192.168.1.1
Name: Wireless_Broadband_Router.home
Address: xxx.yyy.zzz.aaa

The first address, 192.168.1.1, is just the router's internal IP address. The last address (Of course I'm not giving you my IP address. I'm not daft, you know.) is the one we want. xxx.yyy.zzz.aaa is the external address of this machine.

Of course there are many, many, problems with this script.

• So far it only works with the Verzion/Actiontec setup. A friend, running Ubuntu, wasn't able to pick out his gateway machine with the route -n command.
• Some machines can have multiple gateways.
• The Mac's route command doesn't work in the same way as under Linux, so you can't parse out the gateway, even if you have none.
• You might be able to tease the gateway IP address out of the /etc/resolv.conf file. I don't know what happens if you have multiple gateways here. Under Ubuntu, the one numerical gateway is 192.168.1.1, on the Apple it gives me 192.168.1.1 and xxx.yyy.zzz.aaa, my public IP address.

If you try the script, leave a comment below. Let me know your internet setup (gateway/router), if it works, and any tweaks you did to make it work. Maybe we can come up with a universal script.

Now, by all that's geekly, why did we do all of this? That's the subject of an upcoming post, boys and girls …