Networks and services

Hello, good afternoon, what we will see now is a set of examples related to the Nets and Services course, and we will start with a first example, where we will see how to make the IP addresses assignment, how to create subnets, and then, on the next examples, we will see delay calculations, in this case of transmitting a packet on different links, and also of a file transmission.

Let’s start with the first example that you can see on the slides attached on this website, and they suggest the next scenario: a network formed by a Router, where there are two computers connected: B, C and another one here. And they are asking us to assign addresses, to create the different subnets and to assign IP addresses to the different network interfaces of each device. Specifically, they are giving us the next IP addresses range to configure our network.

How do we proceed? Well, first of all we have to identify how many subnets we have together. We can see that we have three, subnet 1: S1, subnet S2, and subnet S3. Remember that a subnet is basically defined by each network interface of the Router. In this case our Router has three network interfaces and therefore it defines three subnets.

Therefore, the first thing we have to do is to be able to name them, to give each subnet a network address. Since we have three subnets, we will need four bits, in other words, 2^2 bits, to be able to name these three subnets. Therefore, we are going to take these bits, from the available bits we have, specifically, we have the mask of the IP addresses range: /28, this means that we have 4 host bits available to define the different subnets, from which we can take two for the different nodes. Therefore we could identify subnet S1 using these two bits, specifically we could identify it with the bits sequence 00, and leave the last two bits to name the different hosts, where these two bits can have four possible values, where this would be the subnet network address and the two bits would be this subnet Broadcast address. Therefore we could identify subnet S1 with the address: 168.1.0/, since we have taken two bits, we have to increase from 28 to 30. In this case the Broadcast address would be 192.168.1, with the two last bits to one, it would be this combination right here, and therefore, we would have the address dot 3, referred to the mask, which is slash 30 (/30).

And they are also asking us to assign each network interface two IP addresses. In this case, we have one here and another one here. We could assign address 1 to computer A, which would be 192.168.1.1 and address 2 on the Router interface. And this way we configured, or named, the network and Broadcast addresses of this subnet and the IPs of each interface.

The proceeding to assign IP addresses to subnet 2 and subnet 3 would be exactly the same and basically we can assign subnet 2 the bits, instead of 00, another combination that could be combination 01 and thus we could find the subnet addresses, Broadcast and IP addresses of each case. In particular, if we only detail the last byte, we would have .1.00000100 for each network address; these would be the last bits that would identify the subnet. For the Broadcast address we would also have: dot 1, four zeros, the same two bits that are put on the subnet address and the two last bits to one (.1.0000XX11). And two other intermediate combinations could be: 01 and 10, which would be for the two network interfaces. In this case, for this one right here we would use combination 01, so we would have the combination 192.168.1. and this in binary would be combination 0101, which corresponds to 5. The value in decimal number 5. And we could assign value 0110 to this IP address from here, which corresponds to value 6.

And with this, we would simply have to repeat the same procedure for subnet S3, to which we could assign the two bits 11, for instance, to identify it, these two bits from here would be 11 and repeat the same proceeding and thus we would finish this example.