Networking: A Beginner’s Guide Fifth Edition- P24:I have run into many people over the years who have gained good even impressive working knowledge of PCs, operating systems, applications, and common problems and solutions. Many of these people are wizards with desktop computers. | Chapter 8 Understanding Networking Protocols 97 which part is the hostid. To see this clearly you need to represent the addresses in binary form Computer IP Address Dec 205 143 60 109 Computer IP Address Bin 11001101 10001111 00111100 01101101 Subnet mask Dec 255 255 255 0 Subnet mask Bin 11111111 11111111 11111111 00000000 The netid of an address defined by the subnet mask is whatever portion of the address has a binary 1 set in the corresponding subnet mask. In the preceding example the netid is the full first three octets the first 24 bits and the hostid is the last octet the last 8 bits . Now you can see why 255 decimal is used so frequently in subnet masks 255 corresponds to having all bits set to 1 in an 8-bit number. NOTE Subnet masks should always use contiguous 1s starting from the left and working to the right. The hostid portion should contain all contiguous 0s working backward from the right to the left. While it is theoretically possible to build subnet masks that have interspersed 1s and 0s it is never done in practice because it would quickly become too complicated to manage properly and because there s no real reason to do so. Also the portion of the hostid that is subnet-masked cannot consist of all 0s or all 1s. While certain implementations of IP do allow all 0s such a configuration is not part of the accepted standard IP rules and thus using such a hostid is risky because some devices on the network might not understand it. Let s now return to the example of the company with three buildings. What if the company could divide a single Class C address so that each building could use its own portion and the routers connecting the buildings would understand which transmissions should be forwarded to the other buildings and which ones should not be Such a configuration is where subnet masks are useful. A subnet mask allows you to borrow some bits from your hostids and then use those bits to create new netids. For the example you would need to borrow .
