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6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM *KZKXSOTOTMZNKGJJXKYYIRGYYH_OTYVKIZOUT The NetID should normally not be all 0s as this indicates a local network. With this in mind, analyze the first octet (‘w’). For class A, the first bit is fixed at 0. The binary values for ‘w’ can therefore only vary between 00000000 2 (0 10 ) and 01111111 2 (127 10 ). 0 is not allowed. However, 127 is also a reserved number, with 127.x.y.z reserved for loop-back testing. In particular, 127.0.0.1 is used to test that the TCP/IP protocol is properly configured by sending information in a loop back to the computer that originally sent the packet, without it traveling over the network. The values for ‘w’ can therefore only vary between 1 and 126, which allows for 126 possible class A NetIDs. For class B, the first two bits are fixed at 10. The binary values for ‘w’ can therefore only vary between 10000000 2 (128 10 ) and 10111111 2 (191 10 ). For class C, the first three bits are fixed at 110. The binary values for ‘w’ can therefore only vary between 11000000 2 (192 10 ) and 11011111 2 (223 10 ). The relationship between ‘w’ and the address class can therefore be summarized as follows. Figure 6.3 IPv4 address range vs class 4[SHKXULTKZ]UXQYGTJNUYZYVKXGJJXKYYIRGYY Note that there are two reserved host numbers, irrespective of class. These are ‘all zeros’ or ‘all ones’ for HostID. An IP address with a host number of zero is used as the address of the whole network. For example, on a class C network with the NetID = 200.100.100, the IP address 200.100.100.0 indicates the whole network. If all the bits of the HostID are set to 1, for example 200.100.100.255, then a broadcast message will be sent to every host on that network. To summarize: • HostID = ‘all zeros’ means ‘this network.’ • HostID = ‘all ones’ means ‘all hosts on this network’ For class A, the number of NetIDs is determined by octet ‘w’. Unfortunately, the first bit (fixed at 0) is used to indicate class A and hence cannot be used. This leaves seven usable bits. Seven bits allow 2 7 = 128 combinations, from 0 to 127. 0 and 127 are reserved; hence only 126 NetIDs are possible. The number of HostIDs, on the other hand, is determined by octets ‘x’, ‘y’ and ‘z’. From these 24 bits, 2 24 = 16 777 218 combinations are available. All zeros and all ones are not permissible, which leaves 16 777 216 usable combinations. For class B, the number of NetIDs is determined by octets ‘w’ and ‘x’. The first bits (10) are used to indicate class B and hence cannot be used. This leaves fourteen usable bits. Fourteen bits allow 2 14 = 16 384 combinations. The number of HostIDs is determined by octets ‘y’ and ‘z’. From these 16 bits, 2 16 = 65 536 combinations are available. All zeros and all ones are not permissible, which leaves 65 534 usable combinations. /TZKXTKZRG_KXVXUZUIURY For class C, the number of NetIDs is determined by octets ‘w’, ‘x’ and ‘y’. The first three bits (110) are used to indicate class C and hence cannot be used. This leaves twenty- two usable bits. Twenty-two bits allow 2 22 = 2 097 152 combinations. The number of HostIDs is determined by octet ‘z’. From these 8 bits, 2 8 = 256 combinations are available. Once again, all zeros and all ones are not permissible which leaves 254 usable combinations. Figure 6.4 Hosts and subnets per class 9[HTKZSGYQY Strictly speaking, one should be referring to ‘netmasks’ in general, or to ‘subnet masks’ in the case of defining netmasks for the purposes of subnetting. Unfortunately, most people (including Microsoft) have confused the two issues and are referring to subnet masks in all cases. For routing purposes it is necessary for a device to strip the HostID off an IP address, in order to ascertain whether or not the remaining NetID portion of the IP address matches the network address of that particular network. Whilst it is easy for human beings, it is not the case for a computer and the latter has to be ‘shown’ which portion is NetID, and which is HostID. This is done by defining a netmask in which a ‘1’ is entered for each bit which is part of NetID, and a ‘0’ for each bit which is part of HostID. The computer takes care of the rest. The ‘1’s start from the left and run in a contiguous block. For example: A conventional class C IP address, 192.100.100.5, written in binary, would be represented in binary as 11000000 01100100 01100100 00000101. Since it is a class C address, the first 24 bits represent NetID and would therefore be masked by 1s. The subnet mask would therefore be: 11111111 11111111 1111111 00000000. To summarize: • IP address: 01100100 01100100 01100100 00000101 • Subnet mask: 11111111 11111111 11111111 00000000 |< NetID >| |< HostID>| The mask, written in decimal dotted notation, becomes 255.255.255.0. This is the so- called default netmask for class C. Default netmasks for classes A and B can be configured in the same manner. Figure 6.5 Default netmasks 6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM Currently IP addresses are issued classless, which means that it is not possible to determine the boundary between NetID and HostID by analyzing the IP address itself. This makes the use of a Subnet Mask even more necessary. 9[HTKZZOTM Although it is theoretically possible, one would never place all the hosts (for example, all 65 534 hosts on a class B address) on a single segment – the sheer volume of traffic would render the network useless. For this reason one might have to revert to subnetting. Assume that a class C address of 192.100.100.0 has been allocated to a network. As shown earlier, a total of 254 hosts are possible. Now assume further that the company has four networks, connected by a router (or routers). Figure 6.6 Before subnetting Creating subnetworks under the 192.100.100.0 network address and assigning a different subnetwork number to each LAN segment could solve the problem. To create a subnetwork, ‘steal’ some of the bits assigned to the HostID and use them for a subnetwork number, leaving fewer bits for HostID. Instead of NetID + HostID, the IP address will now represent NetID + SubnetID + HostID. To calculate the number of bits to be reassigned to the SubnetID, choose a number of bits ‘n’ so that (2 n )–2 is bigger than or equal to the number of subnets required. This is because two of the possible bit combinations of the new SubnetID, namely all 0s and all 1s, are not recommended. In this case, 4 subnets are required so 3 bits have to be ‘stolen’ from the HostID since (2 3 )–2 = 6, which is sufficient in view of the 4 subnets we require. Since only 5 bits are now available for HostID (3 of the 8 ‘stolen’), each subnetwork can now only have 30 HostIDs numbered 00001 (1 10 ) through 11110 (30 10 ), since neither 00000 nor 11111 is allowed. To be technically correct, each subnetwork will only have 29 computers (not 30) since one HostID will be allocated to the router on that subnetwork. The ‘z’ of the IP address is calculated by concatenating the SubnetID and the HostID. For example, for HostID = 1 (00001) on SubnetID = 3 (011), z would be 011 appended to 00001 which gives 01100001 in binary or, 97 10 . /TZKXTKZRG_KXVXUZUIURY Figure 6.7 IPv4 address allocation – 6 subnets on class C address Note that the total available number of HostIDs have dropped from 254 to 180. In the preceding example, the first 3 bits of the HostID have been allocated as SubnetID, and have therefore effectively become part of the NetID. A default class C subnet mask would unfortunately obliterate these 3 bits, with the result that the routers would not be able to route messages between the subnets. For this reason the subnet mask has to be EXTENDED another 3 bits to the right, so that it becomes 11111111 11111111 11111111 11100000. The extra bits have been typed in italics, for clarity. The subnet mask is now 255.255.255.224. Figure 6.8 After subnetting 6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM 6XO\GZK\Y/TZKXTKZ[TOW[K/6GJJXKYYKY If it is certain that a network will never be connected to the Internet, any IP address can be used as long as the IP addressing rules are followed. To keep things simple, it is advisable to use class C addresses. Assign each LAN segment its own class C network number. Then it is possible to assign each host a complete IP address simply by appending the decimal host number to the decimal network number. With a unique class C network number for each LAN segment, there can be 254 hosts per segment. If there is a possibility of connecting a network to the Internet, one should not use IP addresses that might result in address conflicts. In order to prevent such conflicts, either ask an ISP for Internet-unique IP addresses, or use IP addresses reserved for private works. The first method is the preferred one since none of the IP addresses will be used anywhere else on the Internet. The ISP may charge a fee for this privilege. The second method of preventing IP address conflicts on the Internet is using addresses reserved for private networks. The IANA has reserved several blocks of IP addresses for this purpose as shown below: Figure 6.9 Reserved IP addresses Hosts on the Internet are not supposed to be assigned reserved IP addresses. Thus, if the network is eventually connected to the Internet, even if traffic from one of the hosts on the network somehow gets to the Internet, there should be no address conflicts. Furthermore, reserved IP addresses are not routed on the Internet because Internet routers are programmed not to forward messages sent to or from reserved IP addresses. The disadvantage of using IP addresses reserved for private networks is that when a network does eventually get connected to the Internet, all the hosts on that network will need to be reconfigured. Each host will need to be reconfigured with an Internet-unique IP address, or one will have to configure the connecting gateway as a proxy to translate the reserved IP addresses into Internet-unique IP addresses that have been assigned by an ISP. For more information about IP addresses reserved for private networks, refer to RFC 1918. )RGYYRKYYGJJXKYYOTM Initially, the IPv4 Internet addresses were only assigned in classes A, B and C. This approach turned out to be extremely wasteful, as large amounts of allocated addresses were not being used. Not only was the class D and E address space underutilized, but a company with 500 employees that was assigned a class B address would have 65,034 addresses that no-one else could use. Presently, IPv4 addresses are considered classless. The issuing authorities simply hand down a block of contiguous addresses to ISPs, who can then issue them one by one, or break the large block up into smaller blocks for distribution to sub-ISPs, who will then repeat the process. Because of the fact that the 32 bit IPv4 addresses are no longer considered ‘classful’, the traditional distinction between class A, B and C addresses and the implied boundaries between the NetID and HostID can be ignored. Instead, whenever . number of HostIDs, on the other hand, is determined by octets ‘x’, ‘y’ and ‘z’. From these 24 bits, 2 24 = 16 777 218 combinations are available. All zeros and all ones are not permissible,. computer and the latter has to be ‘shown’ which portion is NetID, and which is HostID. This is done by defining a netmask in which a ‘1’ is entered for each bit which is part of NetID, and a ‘0’. considered ‘classful’, the traditional distinction between class A, B and C addresses and the implied boundaries between the NetID and HostID can be ignored. Instead, whenever
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