As mentioned earlier, this is the essential element of IPv4 thinking. It went from one that emphasized the importance of fewer routes in the routing table and consequent router resource conservation to one that emphasized the preservation of host addresses. Thus, the definition of what makes a particular address plan efficient changed over time. Proper aggregation and efficient routing, while still possible, were more often a secondary concern (or a lucky accident). This, in turn, led to the practice of defining and assigning to a link the smallest practical subnet to support immediate and anticipated host counts. Since it allowed for aggregation of networks beyond the 8-bit boundaries of the classful networks, the need for efficient aggregation and routing could be balanced against the need for sufficient host addressing. The CIDR and VLSM methods led to a chain of significant design and operational consequences. Itâs more convenient for humans to represent the subnet mask as either a dotted quad of octets (e.g., 255.255.255.0) or using CIDR notation i.e., â/nnâ notation where the network bits (represented by ânnâ) of an address are appended to the end of the address after a â/â (e.g., 192.0.2.0/24). The 7 bits remaining provide 128 addresses, giving our two new networks 128 total addresses each: 192.0.2.0 to 192.0.2.127Ĭomputers rely on binary operations that require the inclusion of a subnet mask whenever an IPv4 address is represented. Recall that our original class C network had a mask of 255.255.255.0 and 24 bits. Since weâll need 7 bits for host addressing for each segment, that leaves 25 bits for the network, giving us the following subnet mask: 255.255.255.128 According to binary math, the smallest subnet to support 90 servers is provided by 7 bits (though recall that I lose 2 addresses to the network and broadcast addresses):ĩ0 servers using 126 available addresses equates to just a little above 70% utilization, so Iâve still got a little room for growth before Iâll potentially need to renumber. I need enough bits to support a subnet with a host count of at least 90. In my hypothetical network, letâs stipulate that I have two segments that each have 50 servers and that I expect to grow by 25% a year for the next three years: 50 + 3(50 x 0.25) = server count on segment after 3 yearsĪfter three years, neither segment will have more than 90 servers. Converting from binary back to decimal gives us: 192.0.2.0 Youâll notice that the host bits are âzeroed outâ by the operation, while the network bits âpass throughâ the mask. The subnet mask for a class C looks like this: 255.255.255.0 192.0.2.1 to 192.0.2.254Īs with any IPv4 address, the subnet mask must accompany it so that itâs clear what bits are reserved for the network (with the remaining bits set aside for the hosts). As an example, say we had a class C network that we wanted to use to number hosts on various segments: 192.0.2.0īecause itâs a class C, I know that I have 254 usable host addresses. It allowed for any number of the 32 bits of the IPv4 address to be used for the network ID while those bits that remained would define the host addressing. Some other mechanism would be needed to allow the aggregation of any number of smaller subnets into larger ones. In addition, for some network architectures and topologies, even a class C could end up being wasteful if assigned to one segment or interface.Įither way, even with class Cs, the classful addressing approach was simply not sophisticated enough to support sufficient host addressing and efficient routing. Allocating more than 1 class C but fewer than 256 of them meant that there would potentially be many more routing table entries. The 254 host addresses available in a class C network made them ideal for assigning to organizations that had more modest host requirements, especially leaf or stub networks.īut many more organizations (especially small- and medium-sized ISPs) would need multiple class Cs for their host addressing, though perhaps not as much as an entire class B. 24 bits to identify the network, 8 bits for host addressing
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