//Cyber Battlefield/Networking – Nuts & Bolts

Up until this point, we've discussed the general concepts of computer networks and networking protocols. We know that computers communicate over networks by breaking up data, encapsulating it, and sending it out on the network where it will eventually find its way to our intended recipient. How our packets reach their destination or what happens to them after they leave our computers is still a mystery.

In this lesson, we define a network more formally, and discuss how packets move within networks and how they move to other networks.

Move your mouse over the cloud to see what our view of networks will be like by the end of this lesson.
Our current view of the Internet and computer networks

Definition of a Network

We've already talked about the Internet as a network, but the Internet is actually a network of interconnected networks — hundreds of thousands of them! So, what is just a plain old network?
Recall from a previous lecture that a host is just a generic term for a computing device connected to a network endpoint, like the PCs in the diagram at the top of the page. A host could be a laptop, desktop, printer, super computer, server, cellular phone, or even a really fancy refrigerator.
Each network has two special addresses which cannot be used by a host. They are the network address and the broadcast address. The broadcast address is used to send a single packet to all hosts on the network and is characterized by the network address followed by all 1's. A host requesting networking configuration service using DHCP uses the broadcast IP Address, for example.

MAC Addresses and Network-Local Traffic

Hubs, Switches, and Routers

Summary. Link Layer addressing compares to Network Layer addressing as switches compare to routers. A switch forwards packets based on MAC Addresses, while routers forward packets based on IP Addresses. Switches learn about host MAC addresses as a result of the network traffic created by each host. Each MAC address is associated with a connection to the switch, or physical port number, and the association is stored in a table in the switch's internal memory.

Reprise: What it takes to be on a network, and on the Internet

This lesson describes an Ethernet network, and it describes how multiple networks are connected (via routers) to form an internet. The following are two important take-ways:


Sippernet is the colloquial name for the DoD network - isolated from the Internet - that carries IP data that is CONFIDENTIAL or SECRET. SIPRNet packets are encrypted at the Link Layer, allowing CLASSIFIED data to travel across untrusted paths (e.g. from a ship to a military satellite), while also allowing non-secure services to still be used at higher levels of the protocol stack.

The Internet is comprised of links that span the globe. Explore this map of the cables that run across the oceans interconnecting the different continents and bodies of land. There's another version for those that prefer subway maps.