About twice a year the mailman puts your neighbor’s Visa bill in your mailbox, but your network never embarrasses you by sending anything Paris Hilton-related to the kids. How does it do that?

It’s all about IP (Internet Protocol) addressing, and it’s the first thing you need to understand to ask intelligent questions about why you and your kids can’t share the Internet and a printer. This discussion assumes that all your computers either have no software firewall, or that firewalls are all configured correctly (not because that’s realistic, but because it’s an entirely different topic). You may want to turn off any software firewalls while you set up a network behind a router. A router is a bit of a firewall by its very nature. But DO get your software firewall on (and configured) after everything else is working.

What is an IP address and how does it work?

In very broad terms, here’s what you need to know.

You can’t send a letter if you don’t know the address. That’s how networking works. IP addresses are always in this format: xxx.xxx.xxx.xxx. A typical IP address looks like 72.14.207.99, and none of the numbers between the periods will ever be higher than 255. These numbers are called octets.

When you connect your computer to your router (cabled or wireless – it makes no difference) your computer will detect the DHCP server inside your router and ask for an IP address. A DHCP server gives out appropriate IP addresses. Almost without exception, your router’s DHCP server will give your computer an IP that begins with 192.168.y.x. The number in the y spot has to be the same for all computers and printers connected on your network, and each computer must have a unique number in the x position. So a typical network would be:

• Router 192.168.0.1
• Laptop 192.168.0.100
• Kids’ computer 192.168.0.101

The first three octets must be identical. Otherwise these three devices will not talk to each other. 192.168.9.102 could not communicate with this network. There are exceptions to this, but they only apply to large corporate or campus networks and, for the purposes of this article, we’ll keep it simple and say that the third octet must be the same for all of your computers.

These 192s, as network admins call them, are an example of private IP addresses. There are others, such as 10.0.0.x, but you rarely see them in a residential router. Private IPs cannot be routed on the Internet the same way your post office won’t deliver mail without a stamp. Only public IP addresses, the ones your cable modem provides, can be seen from the “outside”.

Each device on your network will have one (and only one) unique IP address. The only exception is your router. It has two addresses, and that’s the point. It has a private IP to communicate with all the devices in your house, and it translates all those private IPs to the one public IP your cable modem provides (sometimes service providers give your devices that are both a modem and router in one – ask them if you’re not sure). So if you take my example network a little farther and add a cable modem, it looks like this:

• Router (public side) 75.73.170.123
• Router (private side) 192.168.0.1
• Laptop 192.168.0.100
• Kids’ computer 192.168.0.101

You know that the public IP could never talk with the others because the first three numbers are not identical. That’s part of what a router does. Communication between the 192s needs no help from the router. But when a 192 address wants to get out of the house to see Paris Hilton’s Chihuahua, it has to use the public IP your cable modem provides. The router makes that happen, and when the picture of Fido comes back, the router makes sure the information is only sent to the exact IP that requested it. Unlike your letter carrier, it never makes a mistake.

Here’s the last part of the whole IP address recipe. Unless you know the IP address of Paris’ Chihuahua picture, your computer will need DNS addresses to get directions.

What is DNS?

Here’s the fastest way to understand DNS. Bring up Internet Explorer or Firefox and type 72.14.207.99 in the address bar (as shown below), then hit enter. You went right to Google, didn’t you?

That’s because your computer understands IP addresses. The problem is they’re hard for you to remember - worse than phone numbers. So DNS servers are essentially a big phone book. When you type google.com into your browser, your computer first goes to a DNS server to look up the IP address.

As part of the information the DHCP server in your router provides when your computer connects, you also get DNS server addresses, usually two of them. If DNS isn’t working right, technically you might still have the Internet. You could still get anywhere you wanted if you knew all the IP addresses you wanted to plug into your browser.

Every computer in your house will use addresses supplied by your router’s DHCP server. All computers will be pointed to the same gateway, which is your router’s address. And they will all have identical IP addresses, except for the last number.

So this is what a healthy network looks like:

Router

• Public IP (WAN side) 75.73.170.123 (from modem)
• Private IP (LAN side) 192.168.0.1

Laptop

• IP 192.168.0.100
• Gateway 192.168.0.1
• Subnet Mask 255.255.255.0
• DNS 68.87.77.130 and 68.87.72.130

Kids’ computer

• IP 192.168.0.101
• Gateway 192.168.0.1
• Subnet Mask 255.255.255.0
• DNS 68.87.77.130 and 68.87.72.130

And when your spouse’s sister visits for a few years next week and connects her laptop, your router’s DHCP server will give her numbers like this:

• IP 192.168.0.102
• Gateway 192.168.0.1
• Subnet Mask 255.255.255.0
• DNS 68.87.77.130 and 68.87.72.130

These computers can all talk directly to each other because they all have the same first three sets of numbers in their private addresses. They use the router to get to the public IP addresses of their DNS servers.

The other thing to note here is the Subnet Mask – this is the number that tells you what octets can, and cannot, be seen by other devices on the same network. As we mentioned earlier, big corporate and campus networks can get much more complex than home networks, so for all intents and purposes, your subnet mask will always be 255.255.255.0.

There is one other type of IP address you might need to know. 169s are kind of a failing grade on the IP address report card. When your computer can’t find a DHCP server and doesn’t know what IP address to use, it gives itself an IP address that starts with 169.254. You will never get to your gateway with that address, which means you will never be able to use the public IP address provided by your cable modem. Your compass isn’t just broken, it’s overboard. Either you aren’t connecting all the way to your wireless signal, DHCP is turned off in your router, or other problems exist.

How do I find my IP address?

In any version of Windows since Win 98*, just go to the Start button, then Run and type “cmd” in the space (without the quotation marks), then hit enter. In the new window just type “ipconfig /all” (no quotation marks) and hit enter again. You’ll see a screen that looks a lot like this:

Scroll to your network card whether it be wireless or not, and all the IP addresses information you just read about is revealed there.

In some versions of Vista you have to add Run to the Start menu. Right click on the Start button, left click Properties. Click the Start Menu tab, then the Customize button by Start Menu. Scroll down and put a check by Run Command, then click OK twice.

If you need to find out your public IP address, you can go to a website such as www.whatismyip.com and it will tell you without much fanfare exactly what the rest of the world thinks your address is.

Tags: networking