Summary: classless classfull subnet-zero cidr vlsm over-network and route Summary

 

Classfull: A class network with no mask information. Supported routing protocols such as r1_1

Classless: Classless, carrying mask information (Network ambiguity after cidr division can be eliminated)

The key points are the classless and classless data forwarding methods.

1. classfull: First matches the primary network number, and then matches the subnet number. If yes, the forwarding is performed. If no, the forwarding is discarded.

If the master network number does not match, the default route is used for forwarding.

2. classless: longest Matching Principle

 

The longest matching principle is used as an example:

Assume that a company has applied for 16 new class C addresses, which are set to 200.200.192.0/24-200.200.207.0/24. Three of them are used in the initial construction phase: 200.200.192.0, 200.200.193.0, 200.200.207.0, A vro connects to the Internet through point-to-point connection.

Then there will be a route table similar to the following on this router:

200.200.192.0/24: e0

200.200.193.0/24: e0

200.200.207.0/24: e1

0.0.0.0/0: s0

The last one is the default path and points to the Internet.

The problem is that the router receives a package whose destination address is 200.200.194.1. What should I do?

According to the above route table, the only route that can match this address is the default route. If there is no other configuration, the router will send it to the Internet. But is that true? 200.200.194.1 is within the scope of the address applied by the company, so it does not exist because the company has not used it. It is certain that this address is not available anywhere on the Internet. The router should discard it. At least on the vro

Ip route 200.200.194.0 255.255.255.0 null0

Ip route 200.200.195.0 255.255.255.0 null0

...

Ip route 200.200.206.0 255.255.255.0 null0

However, there are more than a dozen similar statements.

Use one sentence

Ip route 200.200.192.0 route 240.0 null0

You can.

200.200.192.0 255.255.255.240.0, or 200.200.192.0/20, is a summary of all network segments of the company. When there is data on the 200.200.193.0/24 network segment, because the longest matching principle is used, the data will be forwarded to the correct network segment (according to the above example is e0); when the address does not exist, but within the company address range, the discard statement will be matched, instead of the default route.

 

Ip subnet-zero: Allow all 0 subnets. A long time ago, cisco was no longer used by default. It was rarely used in the r12001 era. However, the fewer rip is used, and now cisco has enabled ip subnet-zero by default.

 

For example, 192.168.1.0/24 is divided into four subnets.

192.168.1.0/26 the two-bit networks borrowed from the fourth eight-bit group are in binary format: 00 000000. All subnet network numbers are 0, so they are all 0 subnets.

 

The reason why the mask can be forwarded by cidr is that the router only forwards different network numbers, rather than the entire IP address.

 

Origin: In the previous classification network, IP Address Allocation separates 32-bit IP addresses from each other by 8-bit. Therefore, the prefix must be 8, 16, or 24 bits. Therefore, the smallest address block that can be allocated has 256 (24-bit prefix, 8-bit host address, 28 = 256) addresses, which is too small for most enterprises. The larger address block contains 65536 (16-bit prefix, 16-bit host, 216 = 65536) addresses, which is too much for large companies. This leads to insufficient use of IP addresses and inconvenience in routing, because a large number of small networks that require separate routing (Class C networks) because it is difficult to implement aggregation routing because it is available in different regions, it adds a lot of burden to routing devices.

 

Classless inter-domian routing ignores A, B, and C network rules and defines A network with the same prefix as A fast route entry; is a method for assigning IP addresses to users and effectively routing IP packets on the Internet.

Advantage: reducing route entries and reducing the size of Route tables

Reduce update overhead

 

The concept of CIDR is not exactly the same as that of the Internet. If the "CIDR" is a knife, the knife is used to cut the "supermesh" and "VLSM" cakes. In fact, CIDR is used when a bid is used.

CIDR includes supernetwork and VLSM

The loose bit is called the supernetwork, for example, 192.168.1.0/22.

The extra bit is called VLSM, for example, 192.168.1.0/28.

Is cidr used for 192.168.1.1/24?

The answer is no! /24 is the default mask and a class IP address.

 

Route aggregation: aggregates a group of routes into a single route broadcast. The final result of Route aggregation and the most obvious advantage is to reduce the size of the route table on the network, and reduce the impact of route update after the subnet network connection is disconnected.

Differences between CIDR and route aggregation: Route aggregation cannot span the primary network, that is, the number of network masks after aggregation is greater than the primary network mask.

If you need to advertise all networks across the border, such as the network is not continuous, you cannot use Route summary. You can disable the auto-collect feature when you use VPC-connected networks (VPC-connected) and VPC-connected networks (VPC-connected.

 

 

This article is from the "one-stop Jiangnan" blog