The Network Layer

The next module in this course was all about the third layer in the TCP/IP model - the network layer. This module went in to more detail about IP addressing and how networks actually communicate with each other. The learning objectives were as follows:

• To describe the IP addressing scheme
• To describe how subnetting works
• To describe subnets by performing basic maths in binary
• To demonstrate how encapsulation works
• To describe how ARP protocols allow different layers of the network to communicate
• To describe how the Internet works
• To understand the basics of routing and routing protocols

I began by learning what an IPv4 address is, the difference between dynamic and static IP addresses and how Dynamic Host Configuration Protocol is used to automatically assign IP addresses to clients. I took a look at an IP datagram header, dissecting it to understand each field & the purpose it serves and finally I was introduced to the concept of encapsulation and how the whole IP datagram is encapsulated as the payload in an Ethernet frame. 

Next I learnt about the different classes of IPv4 addresses and how the network ID and host ID can be determined based on the class the IP address is in. However, I found that this whole system has now been replaced by CIDR, or classless inter-domain routing. I began to understand the principles of Address Resolution Protocol (ARP) and how an ARP table is used to discover hardware addresses for given IP addresses. This is essential for the encapsulation of an IP datagram into an Ethernet frame.

After learning some of the basics I moved on to the subject of subnetting, or splitting a large network into many smaller subnets. I learnt about subnet masks and how the mask determines the size of a subnet i.e. the number of host ID's possible, as well as telling the router which part of the IP address is the subnet ID. I also saw some examples of how subnet masks are added to IP addresses in short hand notation using binary e.g. 9.100.100.100/27 for a subnet mask of 255:255:255:224. By using simple binary math I worked out that this particular example would mean there are 32 possible host addresses available. I then looked a bit further into binary math to understand how subnet masks use AND operators to determine if an IP address exists on the same network by identifying the network and subnet ID portions of an address.

...Once I'd finally got my head around this I began learning about CIDR and how it combines the network and subnet ID's into one, resulting in the elimination of the need for classes and thus rendering the knowledge I learnt about address classes pretty pointless 😂. However it was good to understand the role CIDR plays and how it allows for more versatile network sizes.

Finally, I was taught some basic routing concepts to understand the actual processes behind the sending and receiving of data packets across networks. I learnt how the use of ARP tables and gateway routers facilitate in the sending of information to the correct destinations, and how routing tables store vital network information for the router to use. I also learnt about interior and exterior gateway protocols, as well as the concept of non-routable address space.

Overall I found this module fairly interesting...I took some time to really understand the principles of subnetting and how it can be used to build more efficient networks and it was also good to begin understanding the network layer as a whole and the importance of it's role in networking.

Dave