Network Layer of the OSI Model: Functions, Design & Security

Instructor: Lonny Meinecke

Lonny teaches psychology classes at King University, and has a bachelor's degree in IT and a doctorate in psychology.

In this lesson, we introduce you to the network layer of the OSI model. The network layer is the stage concerned with host to host communication tasks, like protocols, switching, routing, and addressing.

Umm… What is the OSI Model Again?

Just so we are on the same page, the network layer is one of 7 layers in the Open Systems Interconnection (OSI) Model - layer 3 to be exact. Think of the OSI model as a standard way for computers on a network to talk to each other. It's just conceptual really, but it helps us coordinate the transfer of information.

The sent message goes down a vertical stack of layers to the connecting medium (a wired or wireless channel that can pass information) and right back up a similar stack somewhere else. Here's a picture:

Diagram of OSI 7 Layer Model
osi model

What Functions Does the Network Layer Do?

'What kind of work would a network work, if a network could net-work?' (sung to the tune of 'The Woodchuck Song')

Well, the network layer has to figure out where stuff is going to go. It figures out which of the ways between you and your receiver would work out best for this situation. For example, it handles protocols, switching, routing, forwarding, addressing, and error handling.

It can even break up the message into smaller pieces so the whole thing will reassemble at the destination in one meaningful piece (packet switching and sequencing). You might want to think of this fiddling with message bits like a kind of traffic policeman; the network layer's traffic director can tell the sending station to ease up on the throttle a bit if it's too much to handle right now. Here's a picture of store-and-forward packet switching (CRC means cyclic redundancy check. It's a way to verify the data before going to the next step):

Store-and-Forward Packet Switching Visual
store and forward packet switching

The network layer also converts logical addresses (virtual addresses) into physical addresses (real addresses aka MAC addresses, which each of our hardware devices uses as its physical location and identity on the network). So, in addition to a traffic policeman, the network layer is a postman that puts zip codes on electrical messages.

The last thing we will mention that this layer does is accounting (yes, this layer wears many uniforms -- even that of a number-cruncher!). Someone has to keep track of all this to provide billing information!

The Network Layer Handles Traffic, Addressing, and Accounting
traffic mail accounting jobs

What Kinds of Design Issues Does it Have?

Remember that the OSI Model is a conceptualization. It's up to design teams to figure out how to implement it. If you take a course in network design, you will probably run into the following issues at least:

(a) How to handle store-and-forward packet switching - This is about how to break up the message into smaller pieces so the whole thing will reassemble at the destination in one meaningful piece.

(b) What services you are going to provide for the transport layer just above - This lets the layer above know what the network layer can be expected to do - there isn't really a standard, so it's a design issue.

(c) How you're going to implement a connection-based service - Wired connections need to be handled on this layer.

(d) How you're going to implement a connection-less service - Wireless connections also need to be handled here.

(e) Whether you are going to use virtual circuits or datagrams - This is a bit out of our scope, but just realize it's important to know which of these will be used since each has its pros and cons.

Insecurity and Network Anxiety Disorder

All well and good you say, but who's handling security? If you had 3 guesses, would you guess the network layer? Actually, you need to secure anything and everything between trusted parties on a network. Hackers look for any vulnerabilities in the system to exploit. Once a connection can be made (or allowed, say from a botnet infestation, which lets folks in that you don't want in), our steadfast protocols will assume the two parties are who they say they are, which lets them sneak past. Let's cover a few.

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