lecture (06) implementing variable length subnet masks · 2016-12-02 · lecture (06) implementing...
TRANSCRIPT
Lecture (06)Implementing Variable‐LengthSubnet Masks
By:
Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١
• VLSMs provide the ability to include more than one subnet mask within a network and the ability to subnet an already subnetted network address.
• VLSM offers the following benefits:
– More efficient use of IP addresses: Without the use ofVLSMs, companies must implement a single subnet mask within an entire class A, B, or C network number.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٢
• 172.16.0.0/1 6 network address divided into subnetworks using /24 masking.
• 172.16.14.0/24, is further divided into smaller
• subnetworks with the /27 masking, (from 172.16.14.0/27 to 172. 16.14.224/27)
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٣
• one of these smaller subnets, 172. 16.14.128/27, is further divided with the /30 prefix, which creates subnets with only two hosts, to be used on the WAN links (from 172.16.14.128/30 to 172.16.156/30)
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٤
Return to;
VLSM offers the following benefits:
• Greater capability to use route summarization:
– allows more hierarchical levels within an addressing plan and thus allows better route summarization within routing tables
– subnet 172.16.14.0/24 summarizes all of the addresses that are further subnets of 172.16.14.0, including those from subnet 172.16.14.0/27 and from 172.16.14. 128/30.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٥
Return to;
VLSM offers the following benefits:
• Isolation of topology changes from other routers
– when a specific link in the 172.16.27.0/24 domain is flapping, or going up and down rapidly, the summary route does not change.
– Therefore, no router external to the domain needs to keep modifying its routing table because of this flapping activity.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٦
VLSM Calculations
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Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٧
A Working VLSM Example
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Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٨
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٩
• It is important to remember that only unused subnets can be further subnetted.
• In other words, if you use any addresses from a subnet, that subnet cannot be further subnetted.
• In the example, four subnet numbers are used on the LANs. Another unused subnet, 172.16.33.0/26, is further subnetted for use on the WANs.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٠
Route Summarization with VLSM
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١١
• Router A in the figure can route to network 172.16.0.0/16, including all subnets of that network.
• However, if there are other subnets of 172.16.0.0 elsewhere in the network (for example, if 172.16.0.0 was discontiguous), summarizing in this way may not be valid.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٢
• Route summarization, also called route aggregation or supemetting, can reduce the number of routes that a router must maintain by representing a series of network numbers in a single summary address.
• Route summarization is most effective within a subnetted environment when the network addresses are in contiguous blocks in powers of 2.
• For example, 4, 16, or 512 addresses can be represented by a single routing entry because summary masks are binary masks‐ just like subnet masks‐ so summarization must take place on binary boundaries (powers of 2).
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٣
• Routing protocols summarize or aggregate routes based on shared network numbers within the network.
• Classless routing protocols, such as RIPv2, OSPF, and EIGRP, support route summari zation based on subnet addresses, including VLSM addressing.
• Classful routing protocols, such as RIPv 1 and IJGRP, automatically summarize routes on the classful network boundary and do not support summarization on any other boundaries.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٤
Summarizing Within an Octet
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Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٥
• To determine the summary route, the router determines the number of highest‐order bits that match in all of the addresses.
• By converting the IP addresses to the binary format, you can determine the number of common bits shared among the IP addresses.
• the first 21 bits are in common among the IP addresses. Therefore, the best summary route is 172.16.168.0/21.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٦
Summarizing Addresses in aVLSM‐Designed Network•
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٧
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٨
• To allow the router to aggregate the most IP addresses into a single route summary, your IP addressing plan should be hierarchical in nature.
• In the figure, route summarization occurs at the following two levels:
– Router C summarizes two routing updates from networks 172.16.32.64/26 and 172. 16.32.128/26 into a single update, 172. 16.32.0/24.
– Router A receives three different routing updates but summarizes them into a single routing update, then propagates the single update to the corporate network.Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II١٩
Route Summarization Implementation Considerations• Multiple IP addresses must share the same highest‐order bits.
• Routing protocols must base their routing decisions on a 32‐bit IP address and a prefix length that can be up to 32 bits.
• Routing protocols must carry the prefix length (subnet mask) with the 32‐bit IP address.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٢٠
Route Summarization Operationin Cisco Routers• Cisco routers manage route summarization in two ways, as
follows :
• Sending route summaries:
– Routing protocols, such as RIP, IGRP, and EIGRP, perform automatic route summarization across network boundaries
– For OSPF, you must configure manual summarization.
– For EIGRP and RIPv2, you can disable automatic route summarization and configure manual summarization.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٢١
• Selecting routes from route summaries: If more than one entry in the routing table matches a particular destination, the longest prefix match in the routing table is used. Several routes might match one destination, but the longest matching prefix is used.
• For example, if a routing table has different paths to 192.16.0.0/16 and 192. 16.5.0/24, packets addressed to 192.16.5.99 would be routed through the 192.16.5.0/24 path because that address has the longest match with the destination address.
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٢٢
Summarizing Routes in aDiscontiguous Network•
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II٢٣
You can resolve this situation by using RJPv2, OSPF, IS-IS, or EIGRP and not usingsummarization; otherwise, the subnet routes would be advertised with their actual
subnet masks
Thanks,..
See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Fall 2016, Practical App. Networks II
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