Assignment2 (pawan)

Q: - What is difference between between CIDR and VLSM? An: - Here is the difference between CIDR and VLSM: -

VLSM - Variable Length Subnet Masking. Several new methods of addressing were created so that usage of IP space was more efficient. The first of these methods is called Variable-Length Subnet Masking (VLSM). Subnetting had long been a way to better utilize address space. Subnets divide a single network into smaller pieces. This is done by taking bits from the host portion of the address to use in the cre ation of a “sub” network. For  example, take the class B network 147.208.0.0. The default network mask is 255.255.0.0, and the last two octets contain the host portion of the address. To use this address space more efficiently, we could take all eight bits of the third octet for the subnet. One drawback of subnetting is that once the subnet mask has been chosen, the number of hosts on each subnet is fixed. This makes it hard for network administrators to assign IP space based on the actual number of hosts needed. For example, assume that a company has been assigned 147.208.0.0 and has decided to subnet this by using eight bits from the host portion of the address. Assume that the address allocation policy is to assign one subnet per department in an organization. This means that 254 addresses are assigned to each department. Now, if one department only has 20 servers, then 234 addresses are wasted. Using variable-length subnet masks (VLSM) improves on subnet masking. VLSM is similar to traditional fixed-length subnet masking in that it also allows a network to be subdivided into smaller pieces. The major difference between the two is that VLSM allows different subnets to have subnet masks of different lengths. For the example above, a department with 20 servers can be allocated a subnet mask of 27 bits. This allows the subnet to have up to 30 usable hosts on it.

CIDR - Classless Inter-Domain Routing. CIDR is also called super netting. It's an IP addressing scheme that replaces the older system based on

classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP prefix. For example: 172.200.0.0/16 The IP prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP prefix of /12, for example, can be used to address 1,048,576 former Class C addresses. CIDR addresses reduce the size of routing tables and make more IP addresses available within organizations.

Comparing CIDR to VLSM CIDR and VLSM both allow a portion of the IP address space to be recursively divided into subsequently smaller pieces. The difference is that with VLSM, the recursion is performed on the address space previously assigned to an organization and is invisible to the global Internet. CIDR, on the other hand, permits the recursive allocation of an address block by an Internet Registry to a high-level ISP, a mid-level ISP, a low-level ISP, and a private organization’s network. Q: - Why we use sub netting? An: - Subnetting: - When networks were first used all hosts on the network were allocated hostnames such as PC 1 or admin PC but it was soon discovered that it was hard to keep track of all of these names and then routing across a WAN was almost impossible. A RFC (request for comment) was submitted which suggested that we could allocate numbers to hosts on networks instead of hostnames. The current implementation of this scheme is IP version 4 although IPv6 is now replacing this. The idea is that every address would be made up from four groups of eight binary numbers. Each group of eight binary numbers is known as an octet. Because we struggle to write numbers out in binary we usually

convert them into decimal but computers and network devices still see all numbers in binary because they can only recognise on and off signals so either a 0 or a 1. How it Works Each number you allocate to a host on your network will be in groups of four separated by a dot e.g 192.168.1.23. This was working just fine until somebody realised that we needed to identify which parts of the address were for the network and which were for the host on the network. To deal with this we began to add subnet masks to IP addresses. You must use subnet masks even if you only use a basic IP numbering scheme on your network. The rules are that 255.0.0.0 is used for Class A addresses 255.255.0.0 is for Class B and 255.255.255.0 is for Class C addresses. The 255 tells the router that this part of the address is reserved for the network portion of the address. Network Host Host Host Class A 255 0 0 0 Network Network Host Host Class B 255 255 0 0 Network Network Network Host Class 255 C

255

255

0

But it Still Wasn’t Enough When this addressing scheme was devised nobody could have predicted the exponential growth of PC use in companies and then homes all over the world. Instead of only huge companies paying millions for a huge computing device now most people could afford them and they were big enough to put into a box and carry home.

VLSM was devised as an interim measure to prevent us running out of IP addresses. Variable length subnet masking allows you to cut your subnet

down to even smaller portions which mean you can conserve huge amounts of wasted addresses. Instead of being stuck with the below example you could get far more use out of your IP addresses. Class C 192.168.1.0 255.255.255.0 means you can only use the last octet for host addresses on your network. You are not allowed to use 0 at the end because that is your subnet number. You are not allowed to use 255 on the end because that tells the network that it is a broadcast so here are your allowed host numbers. Anything from 192.168.1.1 to 192.168.1.254 so you can only have one large network with 254 hosts on. Imagine 254 hosts all passing huge amounts of traffic across the network.