Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 50 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 459 Tuesday May 20 2003 2 53 PM The Mechanics of Subnetting 459 Table 8-3 Subnetting Chart Subnet Mask Identifier Two Formats Slash Format 25 26 27 28 29 30 Mask 128 192 224 240 248 252 254 255 Bit 1 2 3 4 5 6 7 8 Value 128 64 32 16 8 4 2 1 To determine the number of bits to be used the network designer needs to calculate how many hosts the largest subnetwork requires and the number of subnetworks. For example assume that this requirement is 30 hosts and five subnetworks. To calculate how many bits to reassign consult the Usable Hosts row in Table 8-4. For example for 30 usable hosts 3 bits are required. This also creates six usable subnetworks which satisfies the requirements of this scheme. Again the difference between usable and total hosts is a result of using the first available address as the ID and the last available address as the broadcast for each subnetwork. Classful routing does not provide the capability to use these subnetworks whereas classless routing recovers many of these lost addresses as shown in Table 8-4. This table illustrates the loss of subnets and hosts when you don t use a classless routing protocol. Table 8-4 Subnetting Chart Subnets and Hosts Slash Format 25 26 27 28 29 30 Mask 128 192 224 240 248 252 254 255 Bit 1 2 3 4 5 6 7 8 Value 128 64 32 16 8 4 2 1 Total Subnets 4 8 16 32 64 Usable Subnets 2 6 14 30 62 Total Hosts 64 32 16 8 4 Usable Hosts 62 30 14 6 2 I Page 460 Tuesday May 20 2003 2 53 PM 460 Chapter 8 Routing Fundamentals and Subnets An alternative way to compute the subnet mask and the number of networks is to use the following formulae The number of usable subnets equals 2 to the power of the assigned subnet bits minus 2 2power of bits assigned _ 2 usable subnets For example 23 - 2 6 The number of usable hosts equals 2 to the power of the bits remaining minus 2 2power of bits remaining - 2 usable hosts For example 25 - 2 30 Creating a Subnet To create subnets you must extend the routing portion of .
