Major Nets and Subnet Masks
Every major net has two fields the network field, which uniquely identities the major net, and the host field, which uniquely identifies hosts within the major net. Figure 1-1 illustrates the number of bits in the network and host fields for each class. As mentioned in the previous section, subnetting is the process of dividing a major net into smaller (and generally more useful) subnets. This is accomplished by stealing some bits from the host field of the major net and using those bits to...
Using VLSM for Address Space Efficiency An Example
Suppose Widget, Inc., asks you to subnet one of its class C major nets and tells you it needs the following Two subnets that can support at least 60 hosts Four subnets that can support at least 10 hosts As many subnets as possible that can support two hosts The subnets are needed to support some new additions to its network, as summarized in Table 1-3. Table 1 -3 Subnets Needed by Widget, Inc. Table 1 -3 Subnets Needed by Widget, Inc. As many us possible (use the remaining space) First, you...
Translating Private Addresses into Public Addresses
Cisco routers can dynamically translate private addresses into public addresses, allowing hosts with private addresses to communicate with hosts on the Internet without modification. That is, the privately addressed hosts can function as if they are connected to the Internet. You can configure a router to maintain a pool of public addresses that is smaller than the population of privately addressed hosts. The router then manages the pool and dynamically translates private addresses into public...
Overview of Classless Addressing
Classless addressing (described in RFC 1519) abolishes the idea of traditional classes A, B. and C major nets and the notion of a subnet field. Subnets and major nets do not exist in a classless world instead, there is only a network prefix and a host field. Figure I-7 describes the difference between classful and classless addressing. Figure 1-7 Classful Versus Classless Addressing The length of the network prefix is determined by a prefix mask. The prefix mask is a contiguous series of ones...
Using VLSM Techniques with Classless Addressing
Classless addressing doesn't stop there. You can also break up the space any way you choose by using the same techniques of VLSM. (It's now called variable length network prefixes however, the term VLSM is still commonly used, semantics aside). Remember, there are no more subnets by the true definition of the word because there is no subnet field only a network prefix and its prefix mask. This means there's 110 such thing as a top or bottom subnet, so in using the techniques of VLSM, you can...
R
If you were doing traditional subnetting, you would now be finished, and you would have only two subnets remaining after setting aside Subnets 2 and 3. Clearly, this would not meet Widget, Inc.'s requirements, so start a second round of subnetting. This is where VLSM starts. You do not need Subnets 1 and 4 in their full size (62 host addresses), so subnet them further with a second round of subnetting and a new mask. Perform a second round of subnetting on Subnets 1 and 4 by extending the...
