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.Therefore, each class offers a different amount of bits that can be used for subnetting.Table 2-6demonstrates the tradeoff between the number of subnets and the number of hosts per subnet that can becarved from a Class B IP address.A Class B address uses 16 bits for network number and 16 for hostidentification.As you peruse Table 2-5, you will notice that the fewest number of bits you can allocate tothe network prefix is 2, and the most is 14.The reason for this is simple---a network prefix of 1 bit willonly allow you to define 2 subnet numbers: 0 and 1.The rules for subnetting prevent you from using asubnet address that consists of all 0s or all 1s.Such addresses are reserved! Therefore, a network prefixof 1 bit yields no usable subnets addresses.Similarly, a network prefix of 2 bits yields only two usable subnet addresses.With a 2-bit binary subnetaddress field, the mathematically possible address combinations are 00, 01, 10, and 11.The first and lastcombinations aren't valid, leaving only 01 and 10 for use in identifying subnets.Table 2-5: Subnetting a Class B Address SpaceNumber of Bits in Subnet Mask Number of Usable Subnet Number of Usable HostNetwork Prefix Addresses Addresses, Per Subnet2 255.255.192.0 2 16,3823 255.255.224.0 6 8,1904 255.255.240.0 14 4,0945 255.255.248.0 30 2,0466 255.255.252.0 62 1,0227 255.255.254.0 126 510http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2902.htm (12 of 18) [02/02/2001 11.35.48] Understanding Internetwork Addresses8 255.255.255.0 254 2549 255.255.255.128 510 12610 255.255.255.192 1,022 6211 255.255.255.224 2,046 3012 255.255.255.240 4,094 1413 255.255.255.248 8,190 614 255.255.255.252 16,382 2Obviously, the more bits that are allocated to identifying a subnet number, the fewer remain for hostidentification and vice versa.Class C addresses can also be subnetted.Because a Class C address allocates 24 bits for networkaddressing, only 8 bits remain for apportioning between subnet and host addressing.Table 2-6 presentsthe tradeoffs between subnet and host addressing in a Class C network.Table 2-6: Subnetting a Class C Address SpaceNumber of Bits in Subnet Mask Number of Usable Subnet Number of Usable HostNetwork Prefix Addresses Addresses, Per Subnet2 255.255.192.0 2 623 255.255.224.0 6 304 255.255.240.0 14 145 255.255.248.0 30 66 255.255.252.0 62 2Although Table 2-5 and Table 2-6 demonstrate the tradeoffs between the numbers of possible subnets permask and hosts per subnet, they fall short of actually demonstrating how subnetting works.The best wayto demonstrate subnetting is to actually subnet an IP address, which is done in the next section.A Subnetting ExampleSubnetting is perhaps the most difficult aspect of the IP address architecture to comprehend.This islargely because it only really makes sense when viewed in binary numbers, which isn't very intuitive.You need to subnet the Class C address 193.168.125.0, for example.This is your base address; theone that the Internet would calculate routes to.You need to carve this into six subnets.You would needat least three of the eight host bits to create a unique extended network prefix for each of the six subnets.These addresses would be 001, 010, 011, 100, 101, and 110.The last octet is split: three bits are added tothe network number to form the extended network prefix, and the remaining five are used to identifyhosts.Table 2-7 demonstrates how subnetworks are formed.In Table 2-7, the extended network prefixes (which consists of the IP network address and thesubnetwork address) are in bold.The subnet address is in bold italic.The host addresses are in normaltypeface and separated from the extended network prefix with a hyphen.This makes it easier to see howa basic IP network address can be subdivided into subnetworks.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2902.htm (13 of 18) [02/02/2001 11.35.48] Understanding Internetwork AddressesTable 2-7: Forming SubnetsNetwork Number Binary Address Decimal AddressBase 11000000.101010001.01111101.00000000 193.168.125.0Subnet 0 11000000.101010001.01111101.000-00000 193.168.125.0Subnet 1 11000000.101010001.01111101.001-00000 193.168.125.32Subnet 2 11000000.101010001.01111101.010-00000 193.168.125.64Subnet 3 11000000.101010001.01111101.011-00000 193.168.125.96Subnet 4 11000000.101010001.01111101.100-00000 193.168.125.128Subnet 5 11000000.101010001.01111101.101-00000 193.168.125.160Subnet 6 11000000.101010001.01111101.110-00000 193.168.125.192Subnet 7 11000000.101010001.01111101.111-00000193.168.125.224Each subnetwork number is defined with the first three bits of the last octet.The decimal values of thesedigits are 128, 64, and 32, respectively.The starting IP address (in decimal) for each subnet is presentedin the third column.Not surprisingly, these increment in multiples of 32: the rightmost bit of the subnetnumber.Note Subnets 0 and 7, although mathematically possible to define, are not usable.Their subnet addressesare 000 and 111, respectively.As such, they represent reserved addresses and should not be used toaddress specific subnets.A subnet address of all 0s (regardless of how many 0s) is always reserved foridentifying the subnet itself.A subnet address of all 1s is reserved for broadcasting within the subnet.These subnet addresses are included in Table 2-7 solely to demonstrate the incrementing of the binarysubnet address field from minimum value to maximum value.Hosts in each subnet would be defined by incrementing the remaining five bits in the last octet.There are32 possible combinations of 0s and 1s.The highest and lowest values are reserved, yielding a usablemaximum of 30 hosts per subnet.A device with an IP address of 193.168.125.193 would be thefirst host defined in Subnet 6.Subsequent hosts would be numbered up to 193.168.125.223, atwhich point the subnet would be fully populated.No further hosts could be added.VLSMAlthough subnetting proved a valuable addition to the Internet addressing architecture, it did suffer fromone fundamental limi- tation: You were limited to a single subnet mask for an entire network.Therefore,after you selected a subnet mask (which dictated the number of hosts you could support per subnetnumber) you couldn't support subnets of a different size.Any requirement for larger-sized subnets meantyou had to change the size of the subnet mask for the entire network
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