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Classless vs Classful

What is Classful Routing Protocol?

1. Classful routing protocols do not carry subnet mask information on their routing updates.
   
2. This makes them unsuitable for hierarchical addressing that require Variable Length Subnet Mask (VLSM) and discontiguous network.
   
3. Classless routing protocols do carry subnet mask information on their routing updates.
   
4. RIP v1 and IGRP are classful routing protocols. RIP v2, EIGRP, OSPF, and BGP are classless routing protocol.
   
5. When running a classful routing protocol on a network, make sure you use the same subnet mask everywhere. Otherwise, routing black holes can occur.
   

What is Classless Routing Protocol?

1. Classless routing protocols extend the standard Class A, B, or C IP addressing scheme by using a subnet mask or mask length to indicate how routers must interpret an IP network ID.
   
2. Classless routing protocols include the subnet mask along with the IP address when advertising routing information.
   
3. Subnet masks representing the network ID are not restricted to those defined by the address classes, but can contain a variable number of high-order bits.
   
4. Such subnet mask flexibility enables you to group several networks as a single entry in a routing table, significantly reducing routing overhead.
   
5. Classless routing protocols includes RIP v2 and OSPF, Border Gateway Protocol version 4 (BGP4) and Intermediate System to Intermediate System (IS-IS).
   

 

What is the difference between classful and classless with regards IP addressing, routing, subnetting.

The difference between classful IP addressing and classless IP addressing is in selecting the number of bits used for the network ID portion of an IP address. In classful IP addressing, the network ID portion can take only the predefined number of bits 8, 16, or 24. In classless addressing, any number of bits can be assigned to the network ID.

 Classful protocols cannot carry a network mask as part of the routing advertisement.  This limits the routing protocol to only supporting a single subnetting scheme within the autonomous system.  Additionally, when advertising network addresses between different major network spaces,  the routing protocol will summarize the routing information to the natural class (A, B, C) of the network address.  It must do this since there is no method for one major class network to be updated with the network mask from another major class network.  The best the routing protocol can do is summarized based on the class of the network (derived from the initial bits of the address) and the natural network mask associated with the network class.

A classless protocol has the ability to carry network mask information as part of the routing update.  Since the network mask for each routing entry precisely describes the network being advertised and the address range it covers, a network address range can be subnetted using multiple network masks.
Additionally, it is possible to carry "subnet" information between network address spaces that formerly would have crossed major network boundaries. Essentially, removing the implied network mask based on network class allows the routing protocol to both summarize and subnet an address space based on the network masks associated with each routing table entry.

1. If the networks are classful, the masks of 255.0.0.0, 255.255.0.0 and 255.255.255.0 will be implictly understood for summarizations based on the address (high order bits of the first octet)
   
2. When the network are classless, you have to explicitly declare aggregations, since the classful assumptions fly out the window.
   
3. It always have to include masks on IP route statements when running classless.
   
   

Example :

Classfull IPs are are thing of the past (almost).
Class A addresses: 1.x.x.x to 127.y.y.y and have a /8 net mask.  For example, all the IPs in the 24.x.x.x range are one class A network .
Class B addresses: 128.0.x.x to 191.255.y.y have a /16 net mask.
Class C addresses: 192.0.0.x to 223.255.255.y, /24 net mask.
Of course, they could be subnetted internally in your network.

It's in the routing table that difference appear.  with classfull
addresses, your routing table will contain only one route to whole
classfull networks for networks it's not participating in.  

For example:

  |                                      |
  | e0 --------- s0      s0 --------- e0 |
  |---| router1 |----------| router2 |---|
  |    ---------            ---------    |
  |                                      |
10.1.1.0/26    192.168.1.0/30      172.1.1.0/26

Using "no ip classless" in your routers and isssuing a "show ip routes" on router 1 would yield something like:

Network 10.0.0.0 is subnetted
   10.1.1.0/26         via e0
Network 192.168.1.0 is subnetted
   192.168.1.0/20      via s0
172.1.0.0      via 192.168.1.2

(there is no subnetting for network 172.1.0.0)

Whereas with "ip classless", the routing table would include only a route to 172.1.1.0/26 and would also allow you to use 172.1.1.128/26 on another interface of router1.