In the first part of this article series, I talked about some basic networking hardware such as hubs and switches. In this article, I want to continue the discussion of networking hardware by talking about one of the most important networking components; routers.

Even if you are new to networking, you have probably heard of routers. Broadband Internet connections, such as those utilizing a cable modem or a DSL modem, almost always require a router. A router's job isn't to provide Internet connectivity though. A router's job is to move packets of data from one network to another. There are actually many different types of routers ranging from simple, inexpensive routers used for home Internet connectivity to the insanely expensive routers used by giant corporations. Regardless of a router’s cost or complexity, routers all work on the same basic principles.

That being the case, I'm going to focus my discussion around simple, low budget routers that are typically used to connect a PC to a broadband Internet connection. My reason for doing so is that this article series is intended for beginners. In my opinion, it will be a lot easier to teach you the basics if I am referencing something that is at least somewhat familiar to most people, and that is not as complicated as many of the routers used within huge corporations. Besides, the routers used in corporations work on the same basic principles as the routers that I will be discussing in this article. If you are wanting a greater level of knowledge though, don’t worry. I will talk about the science of routing in a whole lot more detail later in this article series.

As I explained earlier, a router's job is to move packets of data from one network to another. This definition might seem strange in the context of a PC that's connected to a broadband Internet connection. If you stop and think about it, the Internet is a network (actually it's a collection of networks, but that's beside the point).

So if a router's job is to move traffic between two networks, and the Internet is one of those networks, where is the other one? In this particular case, the PC that is connected to the router is actually configured as a very simple network.

To get a better idea of what I am talking about, take a look at the pictures shown in Figures A and B. Figure A shows the front of a 3COM broadband router, while Figure B shows the back view of the same router.

Figure A: This is the front view of a 3COM broadband router

Figure B: A broadband Internet router contains a set of RJ-45 ports just like a hub or switch

As you can see in the figures, there is nothing especially remarkable about the front view of the router. I wanted to include this view anyway though, so that those of you who are unfamiliar with routers can see what a router looks like. Figure B is much more interesting. 

If you look at Figure B, you’ll see that there are three sets of ports on the back of the router. The port on the far left is where the power supply connects to the router. The middle port is an RJ-45 port used to connect to the remote network. In this particular case, this router is intended to provide Internet connectivity. As such, this middle port would typically be used to connect the router to a cable modem or to a DSL modem. The modem in turn would provide the actual connectivity to the Internet.

If you look at the set of ports on the far right, you’ll see that there are four RJ-45 ports. If you think back to the first part of this article series, you’ll recall that hubs and switches also contained large groups of RJ-45 ports. In the case of a hub or switch, the RJ-45 ports are used to provide connectivity to the computers on the network.

These ports work the exact same way on this router. This particular router has a four port switch built in. Remember earlier when I said that a router’s job was to move packets between one network and another? I explained that in the case of a broadband router, the Internet represents one network, and the PC represents the second network. The reason why a single computer can represent an entire network is because the router does not treat the PC as a standalone device. Routers treat the PC as a node on a network. As you can see from the photo in Figure B, this particular router could actually accommodate a network of four PCs. It’s just that most home users who use this type of configuration only plug one PC into the router. Therefore a more precise explanation would be that this type of network routes packets of data between a small network (even if that network only consists of a single computer) to the Internet (which it treats as a second network).

The Routing Process

Now that I've talked a little bit about what a router is and what it does, I want to talk about the routing process. In order to understand how routing works, you have to understand a little bit about how the TCP/IP protocol works.

Every device connected to a TCP/IP network has a unique IP address bound to its network interface. The IP address consists of a series of four numbers separated by periods. For example, a typical IP address looks something like this: 192.168.0.1

The best analogy I can think of to describe an IP address is to compare it to a street address. A street address consists of a number and a street name. The number identifies the specific building on the street. An IP address works kind of the same way. The address is broken into the network number and a device number. If you were to compare an IP address to a Street address, then think of the network number as being like a street name, and at the device number as being like a house number. The network number identifies which network the device is on, and the device number gives the device an identity on that network.

So how do you know where the network number ends and the device number begins? This is the job of the subnet mask. A subnet mask tells the computer where the network number portion of an IP address stops, and where the device number starts. Subnetting can be complicated, and I will cover in detail in a separate article. For now, let's keep it simple and look at a very basic subnet mask.

A subnet mask looks a lot like an IP address in that it follows the format of having four numbers separated by periods. A typical subnet mask looks like this: 255.255.255.0

In this particular example, the first three numbers (called octets) are each 255, and the last number 0. The number 255 indicates that all of the bits in the corresponding position in the IP address are a part of the network number. The number zero indicates that none of the bits in the corresponding position in the IP address are a part of the network number, and therefore they all belong to the device number.

I know this probably sounds a little bit confusing, so consider this example. Imagine that you had a PC with an IP address of 192.168.1.1 and a subnet mask of 255.255.255.0. In this particular case, the first three octets of the subnet mask are all 255. This means that the first three octets of the IP address all belong to the network number. Therefore, the network number portion of this IP address is 192.168.1.x.

The reason why this is important to know is because a router’s job is to move packets of data from one network to another. All of the devices on a network (or on a network segment to be more precise) share a common network number. For example, if 192.168.1.x was the network number associated with computers attached to the router shown in Figure B, then the IP addresses for four individual computers might be:

192.168.1.1 192.168.1.2 192.168.1.3 192.168.1.4

As you can see, each computer on the local network shares the same network number, but has a different device number. As you may know, whenever a computer needs to communicate with another computer on a network, it does so by referring to the other computer’s IP address. For example, in this particular case the computer with the address of 192.168.1.1 could easily send a packet of data to the computer with the address of 192.168.1.3, because both computers are a part of the same physical network.

Things work a bit differently if a computer needs to access a computer on another network. Since I am focusing this particular discussion on small broadband routers that are designed to provide Internet connectivity, let’s pretend that one of the users on the local network wanted to visit the www.brienposey.com Web site. A Web site is hosted by a server. Like any other computer, a Web server has a unique IP address. The IP address for this particular Web site is 24.235.10.4.

You can easily look at this IP address and tell that it does not belong to the 192.168.1.x network. That being the case, the computer that’s trying to reach the Web site can’t just send the packet out along the local network, because the Web server isn’t a part of the local network. Instead, the computer that needs to send the packet looks at its default gateway address.

The default gateway is a part of a computer’s TCP/IP configuration. It is basically a way of telling a computer that if it does not know where to send a packet, then send it to the specified default gateway address. The default gateway’s address would be the router’s IP address. In this case, the router’s IP address would probably be 192.168.1.0.

Notice that the router’s IP address shares the same network number as the other computers on the local network. It has to so that it can be accessible to those computers. Actually, a router has at least two IP addresses. One of those addresses uses the same network number as your local network. The router’s other IP address is assigned by your ISP. This IP address uses the same network number as the ISPs network. The router’s job is therefore to move packets from your local network onto the ISPs network. Your ISP has routers of its own that work in exactly the same way, but that route packets to other parts of the Internet.

In the last part of this article series, I talked about how all of the computers on a network segment share a common IP address range. I also explained that when a computer needs to access information from a computer on another network or network segment, it’s a router’s job to move the necessary packets of data from the local network to another network (such as the Internet).

If you read that article, you probably noticed that in one of my examples, I made a reference to the IP address that’s associated with my Web site. To be able to access a Web site, your Web browser has to know the Web site’s IP address. Only then can it give that address to the router, which in turn routes the outbound request packets to the appropriate destination. Even though every Web site has an IP address, you probably visit Web sites every day without ever having to know an IP address. In this article, I will show you why this is possible.

I have already explained that IP addresses are similar to street addresses. The network portion of the address defines which network segment the computer exists on, and the computer portion of the address designates a specific computer on that network. Knowing an IP address is a requirement for TCP/IP based communications between two computers.

When you open a Web browser and enter the name of a Web site (which is known as the site’s domain name, URL, or Universal Resource Locator), the Web browser goes straight to the Web site without you ever having to enter an IP address. With that in mind, consider my comparison of IP addresses to postal addresses. You can’t just write someone’s name on an envelope, drop the envelope in the mail, and expect it to be delivered. The post office can’t deliver the letter unless it has an address. The same basic concept applies to visiting Web sites. Your computer cannot communicate with a Web site unless it knows the site’s IP address.

So if your computer needs to know a Web site’s IP address before it can access the site, and you aren’t entering the IP address, where does the IP address come from? Translating domain names into IP addresses is the job of a DNS server.

In the two articles leading up to this one, I talked about several aspects of a computer’s TCP/IP configuration, such as the IP address, subnet mask, and default gateway. If you look at Figure A, you will notice that there is one more configuration option that has been filled in; the Preferred DNS server.

Figure A: The Preferred DNS Server is defined as a part of a computer’s TCP/IP configuration

As you can see in the figure, the preferred DNS server is defined as a part of a computer’s TCP/IP configuration. What this means is that the computer will always know the IP address of a DNS server. This is important because a computer cannot communicate with another computer using the TCP/IP protocol unless an IP address is known.

With that in mind, let’s take a look at what happens when you attempt to visit a Web site. The process begins when you open a Web browser and enter a URL. When you do, the Web browser knows that it can not locate the Web site based on the URL alone. It therefore retrieves the DNS server’s IP address from the computer’s TCP/IP configuration and passes the URL on to the DNS server. The DNS server then looks up the URL on a table which also lists the site’s IP

address. The DNS server then returns the IP address to the Web browser, and the browser is then able to communicate with the requested Web site.

Actually, that explanation is a little bit over simplified. DNS name resolution can only work in the way that I just described if the DNS server contains a record that corresponds to the site that’s being requested. If you were to visit a random Web site, there is a really good chance that your DNS server does not contain a record for the site. The reason for this is because the Internet is so big. There are millions of Web sites, and new sites are created every day. There is no way that a single DNS server could possibly keep up with all of those sites and service requests from everyone who is connected to the Internet.

Let’s pretend for a moment that it was possible for a single DNS server to store records for every Web site in existence. Even if the server’s capacity were not an issue, the server would be overwhelmed by the sheer volume of name resolution requests that it would receive from people using the Internet. A centralized DNS server would also be a very popular target for attacks.

Instead, DNS servers are distributed so that a single DNS server does not have to provide name resolutions for the entire Internet. There is an organization named the Internet Corporation for Assigned Names and Numbers, or ICANN for short, that is responsible for all of the registered domain names on the Internet. Because managing all of those domain names is such a huge job, ICANN delegates portions of the domain naming responsibility to various other firms. For example, Network Solutions is responsible for all of the .com domain names. Even so, Network Solutions does not maintain a list of the IP addresses associated with all of the .com domains. In most cases, Network Solution’s DNS servers contain records that point to the DNS server that is considered to be authoritative for each domain.

To see how all this works, imagine that you wanted to visit the www.brienposey.com website. When you enter the request into your Web browser, your Web browser forwards the URL to the DNS server specified by your computer’s TCP/IP configuration. More than likely, your DNS server is not going to know the IP address of this website. Therefore, it will send the request to the ICANN DNS server. The ICANN DNS server wouldn’t know the IP address for the website that you are trying to visit. It would however know the IP address of the DNS server that is responsible for domain names ending in .COM. It would return this address to your Web browser, which in return would submit the request to the specified DNS server.

The top level DNS server for domains ending in .COM would not know the IP address of the requested Web site either, but it would know the IP address of a DNS server that is authoritative for the brienposey.com domain. It would send this address back to the machine that made the request. The Web browser would then send the DNS query to the DNS server that is authoritative for the requested domain. That DNS server would then return the websites IP address, thus allowing the machine to communicate with the requested website.

As you can see, there are a lot of steps that must be completed in order for a computer to find the IP address of a website. To help reduce the number of DNS queries that must be made, the results of DNS queries are usually cached for either a few hours or a few days, depending on how the machine is configured. Caching IP addresses greatly improves performance and

minimizes the amount of bandwidth consumed by DNS queries. Imagine how inefficient Web browsing would be if your computer had to do a full set of DNS queries every time you visit a new page.

Conclusion

In this article, I explained how DNS servers are used to resolve domain names to IP addresses. Although the process that I’ve described sounds fairly simple, it is important to remember that ICANN and top level DNS registrars, such as Network Solutions, use a load balancing technique to distribute requests across many different DNS servers. This prevents any one server from becoming overwhelmed, and eliminates the chances of having a single point of failure.