BROCADE IP PRIMER

Chapter 2: TCP/IPTable 1. Classful RoutingClassFirst FewBits of theAddressFirst OctetRange(in decimal)Subnet MaskNetworkHost perNetworkA 0 0-127 255.0.0.0 128 16,777,214B 10 128-191 255.255.0.0 16,384 65,534C 110 192-223 255.255.255.0 2,097,152 254D 1110 224-239E 1111 240-255Class A. This class is defined by the very first bit in the address being zero. Ifthat first bit is zero, it is a Class A network address. Because the first bit has tobe zero, that means any address from 0.0.0.0 to 127.255.255.255 is consideredto be Class A. Class A's default subnet mask is 255.0.0.0. This meansthat only the first 8 bits are used for the network portion, and the last 24 bitsare used for the host. This means that, in the same Class A network, you couldhave 224 - 2, or 16,777,216 - 2, which equals 16,777,214 hosts. As you cansee, this is a class that allows you to have a few different networks, but eachnetwork could have a lot of hosts in them. To calculate the number of networksyou could create, you would leave out the first bit (as that defines the class).This leaves you with seven bits, and 27 = 128 networks, or the numbers from0 to 127.Class B. This class is defined by the very first bit in the address being a one,but the second bit must be a zero. This means that any address from128.0.0.0 to 191.255.255.255 is considered a Class B. Class B's default subnetmask is 255.255.0.0. This makes it fairly even-handed. You have 16 bitsfor the network portion, and 16 bits for the hosts. This would mean that youcould have 216 - 2, or 65,536 - 2, which would give you 65,534 hosts. Now, fornetworks, you have to leave out the first two bits (as they define the class; aone followed by a zero). This leaves you with 14 (16 - 2) bits, and 214 = 16,384networks. These would be the network portions from 128.0 to 191.255.Class C. This class is defined by the very first bit and the second bit being one,followed by the third bit being a zero. This means that any address from192.0.0.0 to 223.255.255.255 is considered to be a Class C. The defaultClass C subnet mask is 255.255.255.0 (look familiar?). This means that thefirst 24 bits are the network portion, and the last 8 bits are for the hosts. As weshowed in our earlier example, with eight bits you could have 28 - 2, or 256 - 2,which gives you 254 hosts. For networks, however, you have to leave out thefirst three bits (as they define the class; “110” for the first three bits). Thisleaves you with 21 (24 - 3) bits, and 221 = 2,097,152 networks. These wouldbe the network portions from 192.0.0 to 223.255.255. As you can see, thisgives you a lot of networks with fewer hosts per network.44 Brocade IP Primer