Pretty much everyone in the developed world today has at least passing knowledge of how to use the Internet. From a consumer perspective, using the Internet might be as simple as logging your iPad into a wireless network in a coffee shop, then opening a Web browser. IT pros know there's a lot more to it: access points, wireless controllers, a couple of DNS servers, a good firewall, and the all-important upstream Internet connection supplied by an ISP.
However, even for many people in the business, that online connection is the outer limit of our knowledge of how the Internet lives and breathes. Although it's true that unless you actually work for an ISP you probably don't have to know how the Internet sausage is really made, there are tons of ways in which it can be extremely helpful to have a general understanding.
In an age where more and more of an enterprises resources depend on external connectivity -- say, to reach a cloud service -- it's crucial to know how traffic actually gets where it's going on the Internet. Most people know that the Internet is really just an interconnected set of smaller networks, but exactly how are they connected? For example, how did the computer you're using right now know how to get to InfoWorld's website? Knowing that requires a passing understanding of DNS and the BGP routing protocol.
At the beginning of Internet access is DNS
When you fired up your computer or mobile device earlier today, it attached itself to the local network you're on right now. If you're using a PC on a typical enterprise network, your machine likely used a protocol called DHCP (Dynamic Host Configuration Protocol) to request network settings from the network. A DHCP server on the network gave you a few critical pieces of information: An IP address and netmask, a default gateway IP address, and IP addresses for one or more DNS servers.
Getting information from your Internet destination
Now that your computer knows that
www.infoworld.com resolves to 126.96.36.199, it can start sending traffic there. Right away, your computer knows this IP address isn't on the local network, so it can't speak to it directly. Instead, it addresses its traffic to the default gateway's IP address that it learned from the DHCP server. From there, traffic is routed from router to router until it arrives at the server that actually serves up InfoWorld's site. That's where a lot of interesting things happen.
The first stop is the default gateway. Imagine that you're plugged into a large enterprise network and your default gateway is a core router on the network. That core router might be attached to a series of routers and firewalls that make up your company's network. If your company's network is large or complex enough, it probably uses an IGP (Interior Gateway Protocol), such as OSPF or EIGRP, to dynamically share routing information among all the routers on the network. This way, each router on the network only has to know what networks it is directly attached to -- it learns how to get to other networks that aren't adjacent from the routers on the network that are adjacent. As links between them go up or down, the routing tables on each dynamically update to reflect the changes in best paths between them.
However, these routers only know how to get to networks in your company -- an IGP won't teach them anything about how to get to InfoWorld's site. However, the corporate network will have its own default gateway that tells the core router to forward its traffic to the devices guarding the connection to your company's ISP.
After the ISP receives the traffic, it routes it through its network (probably using its own IGP) until it reaches a router that is running an EGP (Exterior Gateway Protocol). The EGP that predominantly runs the Internet is called BGP (Border Gateway Protocol). BGP works by allowing a network -- typically ISPs, but also corporations that connect to more than one ISP -- to advertise which public IP address blocks or prefixes they are responsible for to their peers.