IP Grouping Concepts and Subnetting
The creators of the Internet realized the impracticality of the original network-numbering conventions early on. Computing history shows many examples of people being unable to conceive the idea that computing technology would grow as fast as it has. Needless to say, the Internet would have run out of Class A, B, and C networks long ago if additional addressing features had not been created. Subnetting provided the first significant addressing feature that conserved the global IP address space.
IP subnetting creates vastly larger numbers of smaller groups of IP addresses, compared with simply using Class A, B, and C conventions. The Class A, B, and C rules still exist—but now, a single Class A, B, or C network can be subdivided into many smaller groups. Subnetting treats a subdivision of a single Class A, B, or C network as if it were a network itself. By doing so, a single Class A, B, or C network can be subdivided into many nonoverlapping subnets.
The needs for subnetting are both technical and administrative, as documented in the following list:
• All organizations connected to the Internet (and not using IP address translation) are required to use IP networks registered with the NIC.
• IP protocols enforce the following grouping concept: All hosts in the same group must not be separated by an IP router.
• A corollary to the grouping concept is this: Hosts separated by an IP router must be in separate groups.
• Without subnetting, the smallest group is a single, entire Class A, B, or C network number.
• Without subnetting, the NIC would be woefully short of assignable networks.
• With subnetting, the NIC can assign one or a few network numbers to an organization, and then the organization can subdivide those networks into subnets of more usable sizes.
An example drives these points home. Consider all network interfaces in Figure 6-27, and note which ones are not separated by a router.
Figure 6-27 Backdrop for Discussing Numbers of Different Networks/Subnetworks
Ray 150.1.0.0 150.2.0.0 Hannah
Figure 6-27 Backdrop for Discussing Numbers of Different Networks/Subnetworks
Ray 150.1.0.0 150.2.0.0 Hannah
The design in Figure 6-27 requires six groups, each of which is a Class B network. The four LANs each use a single Class B network. In other words, the LANs attached to routers A, B, C, and D are each a separate network. Additionally, the two serial interfaces composing the point-to-point serial link between routers C and D use the same network because these two interfaces are not separated by a router. Finally, the three router interfaces composing the Frame Relay network with routers A, B, and C are not separated by an IP router and would compose the sixth network. (Note: Other Frame Relay IP addressing options would require one or two more IP network numbers for this physical network.)
However, this design would not be allowed if it were connected to the Internet. The NIC would not assign six separate registered Class B network numbers—in fact, you probably would not even get one Class B because most of the Class B addresses already are assigned. You are more likely to get a couple of Class C networks, and the NIC would expect you to use subnetting.
Figure 6-28 illustrates a more realistic example that uses basic subnetting.
Figure 6-28 Using Subnets
Ray 150.150.1.0 150.15a2.0 Hannah
Figure 6-28 Using Subnets
Ray 150.150.1.0 150.15a2.0 Hannah
- 150.150.4.2
As in Figure 6-27, the design in Figure 6-28 requires six groups. Unlike Figure 6-27, this figure uses six subnets, each of which is a subnet of a single Class B network. This design subnets Class B network 150.150.0.0, which has been assigned by the NIC. The IP network designer has chosen a mask of 255.255.255.0, the last octet of which implies 8 host bits. Because it is a Class B network, there are 16 network bits. Therefore, there are 8 subnet bits, which happen to be bits 17 through 24—in other words, the third octet. Notice that each subnet number in the figure shows a different value in the third octet, representing each different subnet number. In other words, this design numbers or identifies each different subnet using the third octet.
When subnetting, a third part of an IP address appears in the middle of the address—namely, the subnet part of the address. This field is created by "stealing" bits from the host part of the address. The size of the network part of the address never shrinks—in other words, Class A, B, and C rules still apply when defining the size of the network part of an address. Figure 6-29 shows the format of addresses when subnetting.
Figure 6-29 Address Formats When Subnetting Is Used
|
Network |
Subnet |
Host |
Class A |
|
|
16 |
16-x |
X |
||
|
Network |
Subnet |
Host |
Class B |
|
|
24 |
8-x x |
|||
|
Network |
Class C |
|||
Three portions of the address now exist: network, subnet, and host. Class rules determine the size of the network part. The subnet mask determines the size of the host part—the number of bits of value 0 in the subnet mask defines the number of host bits. The remaining bits define the size of the subnet part of the address. For example, a mask of 255.255.255.224, used with a Class C network, implies five host bits. (The mask can be more easily converted to decimal using the table in Appendix B, "Decimal to Hexadecimal Binary Conversion Table.") The mask has five binary 0s at the end, implying five host bits. As shown in Figure 6-29, a Class C network has 24 network bits. That leaves three subnet bits—not many, but it still provides more groupings than if this Class C network was not subnetted at all!
The number of host bits implies how many valid host addresses exist in the subnet; 2hostbits minus 2 special reserved cases is the formula. Similarly, the number of subnet bits implies the number of valid subnets of a network, assuming that the same mask is used on all subnets; 2subnetbits is the formula. Two special subnets, the "zero subnet" and the "broadcast subnet," were reserved in years past but are now usable. However, when you get a test question about the number of possible subnets, the "right" answer is 2subnetbits - 2.
Continue reading here: Binary View of Subnetting
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