3Com 3C888 User Guide - Page 141

What is a network? 141 reserved for other purposes). By contrast, a Class A network's IP address allocates eight bits for the network number and 24 bits for the local address, allowing for a possible combination of 16,777,214 (224-2) local addresses. To return to the postal analogy, just as a street address refers to the set location or residence itself and not to the occupant, who may change over time, an IP address refers to a device's location and not the device itself. Thus, for example, if a PC is relocated to another area of the same network, it does not retain the same IP address. What is a Subnet Mask? A subnet mask is a pattern of 32 binary digits (ones or zeroes) which is transmitted alongside an IP address to instruct certain routing devices (namely, internal routers within private organizations, as opposed to public, Internet routers) to read or interpret the IP address in an alternate way. Specifically, a subnet mask instructs a routing device to take the host number portion of the IP address and further subdivide it into two smaller segments: a subnet number and an even shorter host number. Where this subdivision occurs depends upon the pattern of ones vs. zeros in the subnet mask. That is, within the string of the subnet mask's 32 bits, the point at which the ones end and the zeroes begin is the point at which the subdivision is made. The resulting subnet number is then added to the end of the original network number, such that a new, longer network number is temporarily derived. The mechanism of a subnet mask is analogous to a colored lens used in an optician's test to blot out one half of a two-toned color pattern in order to reveal a second, sub-pattern. Just as the colored lense is used to "mask" all of one color, the string of ones in a subnet mask is used to mask any bit in the IP address that corresponds to a one in the subnet mask, when the address and mask are matched up against one another. This idea is illustrated in Figure 70, which shows a subnet mask in a Class C network. Subnet Mask Applied to IP Address in a Class C Network Network Number (24 bits) Host Number (8 bits) IP Address 11 0 0 0 0 0 0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 1 (as read by external router) Subnet Mask 11111111 . 11111111 . 11111111 . 111 0 0 0 0 0 IP Address (as read by internal router, through "lense" of Subnet Mask) 11 0 0 0 0 0 0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 1 (New) Network Number (27 bits) Subnet Number (3 bits) (New) Host Number (5 bits) Bits "masked" by ones are read as part of (New) Network Number Bits left unmasked by zeroes are read as part of (New) Host Number Figure 70 Subnet Masking