计算机网络第4章：OSI层次：网络层.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * VLSM Advantages: Efficient use of IP addresses Better route aggregation Support VLSM Routing Protocol: Open Shortest Path First (OSPF) Integrated Intermediate System to Intermediate System (Integrated IS-IS) Enhanced Interior Gateway Routing Protocol (EIGRP) RIP v2 Static routing. A Waste of Space In the past, it has been recommended that the first and last subnet not be used. But we can used the Subnet 0 from Cisco IOS ver12.0. From IOS ver12.0, the Cisco router use subnet zero by default. command: router(config)#no ip subnet-zero A Waste of Space 192.168.187. 000 ***** 24 (1110 0000) Host-id subnet-id How to use the VLSM VLSM: An Example VLSM: An Example A class C address of /24 has been allocated. Perth, Sydney, and Singapore have a WAN connection to Kuala Lumpur (KL). Perth requires 60 hosts. KL requires 28 hosts. Sydney and Singapore each require 12 hosts. To calculate VLSM subnets and the respective hosts allocate the largest requirements first from the address range. Requirements levels should be listed from the largest to the smallest. VLSM: An Example Step 1 In this example Perth requires 60 hosts. Use 6 bits since 26 – 2 = 62 usable host addresses. Thus 2 bits will be used from the 4th Octet to represent the extended-network-prefix of /26 and the remaining 6 bits will be used for host addresses. Applying VLSM on address /24 gives: 0 hh hhhh /26 92 (1100 0000) /24（Subnetted） /26 (0000 0000) Perth (60 hosts) /26 (Network Address) /26 /26 …… 1/26 2/26 3/26(Broadcast Address) 4/26 (0100 0000) Unused （Subnetted） 28/26 (1000 0000) Unused 92/26? (1100 0000) Unused See next slide VLSM: An Example Step 2 KL requires 28 hosts. The next available address after 3/26 is 4/26. Since 28 hosts are required, 5 bits will be