# Classful IPv4 networks
## Learning goals

* understand that an IP network as an overlay network is an abstract thing that is not directly reflecting the hardware settings
* understand the notion of an IPv4 address and its components like network and host part
* understand why MAC addresses do not fulfill the requirements of IP addresses.
* get introduced to the notion of an IP router / gateway

## Script

As we have already seen with the Ethernet we can build Local Area Network and those networks will work as the autonomous systems. But there is an essential need to interconnect these networks by creating an overlay network. In order to create this overlay network we need to introduce concept that is called classfull network or classfull IP network. For them we need to introduce the concept of IP address.
For example, in one network each of the computers even thought it already has a MAC Address gets another address which is called an IP Address or IPv4 Address. - this IP address is basically 32 bit integer, so it consists of 4 bytes an each byte is denoted as the number separated with dot (67.68.50.1) and it’s just to make it easier to read.
- what this also means is that each of those number between the dots can be between 0 and 255.
- IP address is supposed to uniquely identify each host on the network and in global Internet. All the IP addresses in the network start with the same number (the same prefix). It’s called network prefix. This prefix is only 1 byte but there are networks with longer prefix. This is a very simple principle of classfull network. So now with all IP addresses in the same network starting with the same network part (67 for example) we’ve got one special IP address (e.g. 67.0.0.0). This IP address will never be assigned to any host in the network. This is reserved to identify network itself. By doing this we introduce some kind of hierarchy. So even so the Internet is still decentralized we need this hierarchy for the routing algorithm to work. Unlike the MAC addresses we can make sure that all IP addresses in the same network start with the same prefix. It will be impossible with MAC addresses due to the fact that MAC address consists of some parts identifying the vendor and just a serial number. What can happen is that similar MAC addresses of Ethernet devices will exist in networks all over the world.
Let’s move to another network. It’s has a larger network part of IP address (141.71.0.0). So in order to interconnect this network and the previous one (67.0.0.0), we need to introduce one further computer which will have 2 network devices. Each network device will be in one of those networks. So what can happen now? If packages are in 67-network and their destination is 141.71-network they can just go to this special computer and it could forward them to the other network. A routing table mechanism is required. After you have connected those 2 networks you can also connect 141.71-network with the similar mechanism by introducing one other computer with 205.34.55-network. And once you have done this packages could actually travel from the 67-network over the 141.71-network to the 205.34.55-network. Those introduced computers are called routers. --Jane Kruch (discuss • contribs) 10:33, 31 October 2013 (UTC)