A friend contacted me with a networking problem the other day that I thought I would share with you. Maybe you have experienced it as well. And you know the definition of experience right? Experience is what you get when you didn’t get what you wanted!

My friend and his wife had gone wireless. They bought new laptops with built-in wireless (WIFI) and wanted to begin to roam around the homeplace untethered by LAN cables while connected to the Internet.

So in addition to the laptops they came home with a LinkSys wireless
router to provide the necessary access point for the laptops. The guy at the computer store said it would work like a charm. All they had to do was plug in the wireless router and go.

After rebooting, powering off and on, and calling the computer store
all with no success, they figured they had nothing to lose by calling me. I haven’t figured out whether I’m flattered or insulted.

Nonetheless, I was able to help because I had experience (see definition above).

Let’s start with a little background. As you probably know, to send and receive information on the Internet, your computer has to have an Internet Protocol (IP) address. This IP address is a part of the Internet’s Transmission Control Protocol (TCP). These addresses, which uniquely identify devices on the Internet, are given out in blocks to Internet Service Providers (ISPs) by the Internet Assigned Numbers Authority (IANA).

Your ISP then redistributes these addresses to its customers. You, in other words!

I haven’t heard of any ISP that gives out multiple IP addresses per account.

Usually if you have one digital subscriber line (DSL) or one cable-TV Internet connection you get one IP address. And even then, the IP is frequently not permanently assigned to you.

The ISP will let you use it while you’re online and then give it to someone else when you disconnect (i.e. turn off your DSL or cable modem). This process is called Dynamic Host Configuration Protocol (DHCP). Remember this DHCP. It will show up later!

So how is it that at your home you’ve been able to have more than one computer with only one IP address?

Well two things come into play. First the IANA has reserved the following three blocks of the IP address space for private internets:

10.0.0.0 – 10.255.255.255 (10/8 prefix)
172.16.0.0 – 172.31.255.255 (172.16/12 prefix)
192.168.0.0 – 192.168.255.255 (192.168/16 prefix)

Secondly there’s another protocol called Network Address Translation (NAT) involved.

The private IP address mentioned can be used freely and the Internet system should ignore any those addresses if they wander out on the information superhighway.

But your local router listens for those addresses. And if there are any packets, as they’re sometimes called, with a private (or internal) IP address headed out onto the Internet, your router will substitute its own IP address that it acquired from your ISP (this is the Network Address Translation).

When information returns your router will get it first and keep track of where to send it back to you via your private (or internal) IP address.
Your router is able to do this even if you have multiple computers on your private (or internal) network.

Now, getting back to our problem. You may have noticed that your wired PC gets one of these NAT private addresses such as 192.168.1.100 when you turn it on. Routers request a real address from your ISP (many times via DHCP). And then your router may give you an private IP address via its own DHCP server. When it gives you the IP address it also tells you to send your packets to it first. It becomes your gateway so that it can translate the internal private IP address to the real IP address. So the gateway IP address many (but not all) routers choose by default is 192.168.1.1 and especially LinkSys routers.

But here’s what happened to my friend. They already had a wired router (a Netopia) that was getting a real Internet address and using a private internal gateway IP address of 192.168.1.1 for the computers on the wired network. When the new LinkSys wireless router was plugged into the Netopia, it asked for an IP address via DHCP. It was give an IP address of 192.168.1.114 by the Netopia. And then the LinkSys set itself up as gateway IP address 192.168.1.1 for the wireless PCs that connected.

Now you might think this would work since there were two separate private networks in fact. But as it turns out, routers are pretty smart, but apparently not that smart. The LinkSys wireless router received outbound packets at gateway IP address 192.168.1.1 but got mixed up trying to forward them onto the Netopia router which also had a gateway IP address of 192.168.1.1 that it was using.

The solution? Using the LinkSys browser-based configuration utility I changed the default gateway IP address to 192.168.2.1 for the LinkSys.

Now I ignored some IP addressing issues such as subnets and masks.
So if you’re looking for more information or more details, try searching for
“IP addressing”, “private IP network”, “NAT protocol”, or “DHCP”.

I hope my experience saves you from getting some experience!