Classless and Classful Routing
Classful vs Classless Routing v1.10 – Aaron Balchunas
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- Classful vs. Classless Routing Classful vs Classless routing protocols Classful routing protocols do not send subnet mask information with their routing updates. A router running a classful routing protocol will react in one of two ways when receiving a route: • If the router has a directly connected interface belonging to the same major network, it will apply the same subnet mask as that interface. • If the router does not have any interfaces belonging to the same major network, it will apply the classful subnet mask to the route. Belonging to same “major network” simply indicates that they belong to the same “classful” network. For example: • 10.3.1.0 and 10.5.5.0 belong to the same major network (10.0.0.0) • 10.1.4.5 and 11.1.4.4 do not belong to the same major network • 192.168.1.1 and 192.168.1.254 belong to the same major network (192.168.1.0) • 192.168.1.5 and 192.167.2.5 do not belong to the same major network. Take the following example (assume the routing protocol is classful):
If Router B sends a routing update to Router A, it will not include the subnet mask for the 10.2.0.0 network. Thus, Router A must make a decision. If Router A has a directly connected interface that belongs to the same major network (10.0.0.0), it will use the subnet mask of that interface for the route. For example, if Router A has an interface on the 10.4.0.0/16 network, it will apply a subnet mask of /16 to the 10.2.0.0 network. If Router A does not have a directly connected interfacing belonging to the same major network, it will apply the classful subnet mask of /8. This can obviously cause routing difficulties. When using classful routing protocols, the subnet mask must remain consistent throughout your entire network.
*** All original material copyright © 2006 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be
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- Classful vs. Classless Routing Classful vs Classless routing protocols Classful routing protocols do not send subnet mask information with their routing updates. A router running a classful routing protocol will react in one of two ways when receiving a route: • If the router has a directly connected interface belonging to the same major network, it will apply the same subnet mask as that interface. • If the router does not have any interfaces belonging to the same major network, it will apply the classful subnet mask to the route. Belonging to same “major network” simply indicates that they belong to the same “classful” network. For example: • 10.3.1.0 and 10.5.5.0 belong to the same major network (10.0.0.0) • 10.1.4.5 and 11.1.4.4 do not belong to the same major network • 192.168.1.1 and 192.168.1.254 belong to the same major network (192.168.1.0) • 192.168.1.5 and 192.167.2.5 do not belong to the same major network. Take the following example (assume the routing protocol is classful):
If Router B sends a routing update to Router A, it will not include the subnet mask for the 10.2.0.0 network. Thus, Router A must make a decision. If Router A has a directly connected interface that belongs to the same major network (10.0.0.0), it will use the subnet mask of that interface for the route. For example, if Router A has an interface on the 10.4.0.0/16 network, it will apply a subnet mask of /16 to the 10.2.0.0 network. If Router A does not have a directly connected interfacing belonging to the same major network, it will apply the classful subnet mask of /8. This can obviously cause routing difficulties. When using classful routing protocols, the subnet mask must remain consistent throughout your entire network.
*** All original material copyright © 2006 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be