I understand that for classful networks in EIGRP we do not need a subnet mask. Why is this? When I do a show ip protocols the routes appear without a subnet mask which looks somewhat odd.

If I have a network and just introduce the network into the EIGRP routing protocol as, how will EIGRP know that it is and not say

Or does EIGRP understand the network address from the host addresses assigned to the router's DC interfaces?


  • 2
    What is the point of this question? Classful networking has not been used by anyone with any knowledge in over 20 years. This is like asking how to write a program to run efficiently on an 8086 processor.
    – YLearn
    Apr 22, 2015 at 12:19
  • @YLearn Does Stuart Smith's answer also answer your comment? Apr 23, 2015 at 13:44
  • 2
    @ToddWilcox, no. My point is that no one should be considering "classful networks" in today's day and age and shouldn't use classful defaults. Using classful networking, there is no way to have as a valid network.
    – YLearn
    Apr 23, 2015 at 22:18

1 Answer 1


By default EIGRP uses a classful network, meaning that it assumes that an address in the class A range with have an 8-bit subnet mask. There is no automatic process of checking other than the assumption that if you do not specify the subnet mask in wildcard mask format ( host mask ) then you want to use the default subnet mask for the IP range you've specified.


As @Todd Wilcox pointed out below, each address range has a class, originally specified in RFC791 ( https://www.ietf.org/rfc/rfc791.txt ), excluding special use addresses ( https://www.rfc-editor.org/rfc/rfc3330 ).


Class      Sig Bits      Network Mask  Range

A          0             8      -

B         10             16   -

C        110             24   -

D       1110             undefined -

E       1111             undefined -

So converting the first octet ( first group of decimal digits in an IP address: ) into binary gives us 11000000, matching the MSB ( most significant bits ) of a C class address, implying a default subnet mask of /24 bits or


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