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.)2.1.2.1 Types of SubnettingThere are two types of subnetting: static and variable length.Variable length is themore flexible of the two.Which type of subnetting is available depends upon therouting protocol being used; native IP routing supports only static subnetting, asdoes the widely used RIP protocol.However, RIP Version 2 supports variablelength subnetting as well.See 3.3.1, Routing Information Protocol (RIP) onpage 106 for a description of RIP and RIP2.Chapter 3, Routing Protocols onpage 95 discusses routing protocols in detail.Static Subnetting: Static subnetting means that all subnets in the subnettednetwork use the same subnet mask.This is simple to implement and easy tomaintain, but it implies wasted address space for small networks.For example, anetwork of four hosts that uses a subnet mask of 255.255.255.0 wastes 250 IPChapter 2.Internetworking and Transport Layer Protocols 31 addresses.It also makes the network more difficult to reorganize with a new subnetmask.All hosts and routers are required to support static subnetting.Variable Length Subnetting: When variable length subnetting is used, thesubnets that make up the network can use different subnet masks.A small subnetwith only a few hosts needs a subnet mask that accommodates only these fewhosts.A subnet with many hosts attached may need a different subnet mask toaccommodate the large number of hosts.The possibility to assign subnet masksaccording to the needs of the individual subnets will help conserve networkaddresses.Also, a subnet can be split into two parts by adding another bit to thesubnet mask.Other subnets in the network are unaffected by the change.Variable length subnetting allows you to divide the network so that it is possible todefine adequate hosts for each subnet by changing the subnet mask for eachnetwork.This can be achieved by configuring the routers accordingly.Please notethat not every host and router supports variable length subnetting.With staticsubnetting each subnet has the same number of hosts.If it is required to havedifferent numbers of hosts for each network, then variable length subnetting shouldbe used.Only networks of the size needed will be allocated and routing problems will besolved by isolating networks with routers that support variable subnetting.A hostthat does not support this kind of subnetting would have to route to a router thatsupports variable subnetting.Mixing Static and Variable Length Subnetting: At first sight, it appears that thepresence of a host that only supports static subnetting would prevent variablelength subnetting from being used anywhere in the network.Fortunately this is notthe case.Provided that the routers between subnets with different subnet masksare using variable length subnetting, the routing protocols employed are able tohide the difference between subnet masks from the hosts in a subnet.Hosts cancontinue to use basic IP routing and offload all of the complexities of the subnettingto dedicated routers.2.1.2.2 A Static Subnetting ExampleRecall that an IP address consists of the pair.For example, let us take a class A network; the address format is shown inFigure 8:1 2 301 8 6 4 10 netID hostIDClass AFigure 8.IP - Class A Address without SubnetsLet us use the following IP address:1 1 1 11 1 11 1 a 32-bit address9 67 38 1 decimal notation (9.67.38.1)9.67.38.1 is an IP address (class A) having9 as the67.38.1 as the32 TCP/IP Tutorial and Technical Overview Subnets are an extension to this by considering a part of the to bea subnetwork address.IP addresses are then interpreted as.For example, you may wish to choose the bits from 8 to 25 of a class A IP addressto indicate the subnet addresses, and the bits from 26 to 31 to indicate the actualhost addresses.Figure 9 shows the subnetted address that has thus been derivedfrom the original class A address:1 2 301 8 6 4 1Class A host0 netID subnet numberIDSubnetFigure 9.IP - Class A Address with Subnet Mask and Subnet AddressWe normally use a bit mask, known as the subnet mask, to identify which bits ofthe original host address field should indicate the subnet number.In the aboveexample, the subnet mask is 255.255.255.192 in decimal notation (or 1111111111111111 11111111 11000000 in bit notation).Note that, by convention, theis masked as well [ Pobierz całość w formacie PDF ] |
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