Al-Mustansiriyah University Fourth Year ( )

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1 What subnet and broadcast address is IP address , (/27) a member of? Answer: The interesting octet is the fourth octet =32 block size. Because 32+32=64 and 33 is between 32 and 64. So the answer would be the subnet. The broadcast is , since is the next subnet. Subnetting Class A Addresses Class A subnetting is not performed any differently than Classes B and C, but there are 24 bits to play with instead of the 16 in a Class B address and the 8 in a Class C address. Let s start by listing all the Class A masks: (/8) (/9) (/20) (/10) (/21) (/11) (/22) (/12) (/23) (/13) (/24) (/14) (/25) (/15) (/26) (/16) (/27) (/17) (/28) (/18) (/29) (/19) (/30) Subnetting Practice Examples: Class A Addresses Practice Example #1A: (/16) Class A addresses use a default mask of , which leaves 22 bits for subnetting since you must leave 2 bits for host addressing. The mask with a Class A address is using 8 subnet bits. _ Subnets? 2 8 = 256. _ Hosts? = 65,534. _ Valid subnets? What is the interesting octet? = 1. 0, 1, 2, 3, etc. (all in the second octet). The subnets would be , , , , etc., up to Broadcast address for each subnet? 36

2 The following table shows the first two and last two subnets, valid host range, and broadcast addresses for the private Class A network: Subnet First host Last host Broadcast Practice Example #2A: (/20) gives us 12 bits of subnetting and leaves us 12 bits for host addressing. _ Subnets? 2 12 = _ Hosts? = _ Valid subnets? What is your interesting octet? = 16. The subnets in the second octet are a block size of 1 and the subnets in the third octet are 0, 16, 32, etc. _ Broadcast address for each subnet? The following table shows some examples of the host ranges the first three and the last subnets: Subnet First host Last host Broadcast Practice Example #3A: (/26) Let s do one more example using the second, third, and fourth octets for subnetting. _ Subnets? 2 18 = 262,144. _ Hosts? = 62. _ Valid subnets? In the second and third octet, the block size is 1, and in the fourth octet, the block size is 64. _ Broadcast address for each subnet? The following table shows the first four subnets and their valid hosts and broadcast addresses in the Class A mask: Subnet First host Last host Broadcast

3 The following table shows the last four subnets and their valid hosts and broadcast addresses: Subnet First host Last host Broadcast Limits to the Class-based System Not all organizations' requirements fit well into one of these three classes. Classful allocation of address space often wasted many addresses, which exhausted the availability of IPv4 addresses. For example, a company that had a network with 260 hosts would need to be given a class B address with more than 65,000 addresses. Even though this classful system was all but abandoned in the late 1990s, you will see remnants of it in networks today. For example, when you assign an IPv4 address to a computer, the operating system examines the address being assigned to determine if this address is a class A, class B, or class C. The operating system then assumes the prefix used by that class and makes the appropriate subnet mask assignment. Another example is the assumption of the mask by some routing protocols. When some routing protocols receive an advertised route, it may assume the prefix length based on the class of the address. Classless Addressing The system that we currently use is referred to as classless addressing. With the classless system, address blocks appropriate to the number of hosts are assigned to companies or organizations without regard to the unicast class. Variable Length Subnet Masks (VLSMs) VLSM is a way to take one network and create many networks using subnet masks of different lengths on different types of network designs. Neither RIPv1 nor IGRP routing protocols have a field for subnet information, so the subnet information gets dropped. What this means is that if a router running RIPv1 has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is called classful routing, and RIPv1 and IGRP are both considered classful routing protocols. 38

4 Classless routing protocols, however, do support the advertisement of subnet information. Therefore, you can use VLSM with routing protocols such as RIPv2, EIGRP, and OSPF. The benefit of this type of network is that you save a bunch of IP address space with it. As the name suggests, with VLSMs we can have different subnet masks for different router interfaces. Look at Figure 3.3 to see an example of why classful network designs are inefficient. Looking at this figure, you ll notice that we have two routers, each with two LANs and connected together with a WAN serial link. In a typical classful network design (RIP or IGRP routing protocols), you could subnet a network like this: = Network (/28) = Mask Figure 20: Typical classful network. Our subnets would be 0, 16, 32, 48, 64, 80, etc. This allows us to assign 16 subnets to our internetwork. But how many hosts would be available on each network? Well, as you probably know by now, each subnet provides only 14 hosts. This means that each LAN has 14 valid hosts available one LAN doesn t even have enough addresses needed for all the hosts! But the point-to-point WAN link also has 14 valid hosts. It s too bad we can t just nick some valid hosts from that WAN link and give them to our LANs! 39

5 All hosts and router interfaces have the same subnet mask again, this is called classful routing. And if we want this network to be more efficient, we definitely need to add different masks to each router interface. VLSM Design Let s take Figure 20 and use a classless design which will become the new network shown in Figure 21. In the previous example, we wasted address space one LAN didn t have enough addresses because every router interface and host used the same subnet mask. Not so good. What would be good is to provide only the needed number of hosts on each router interface. To do this, we use what are referred to as Variable Length Subnet Masks (VLSMs). Now remember that we can use different size masks on each router interface. And if we use a /30 on our WAN links and a /27, /28, and /29 on our LANs, we ll get 2 hosts per WAN interface, and 30, 14, and 8 hosts per LAN interface nice! This makes a huge difference not only can we get just the right amount of hosts on each LAN, we still have room to add more WANs and LANs using this same network! Figure 21: Classless network design. 40