CCNA 3.2.b: Network mask

Overview:

A prefix is simply the network address. A prefix-length is simply a shorthand way to express a network mask using CIDR notation.

 

Study Notes:

Show IP Route Image

  • A network mask may also be called a subnet mask or netmask for short
  • Let’s read a few of the entries in the routing table and determine their network masks:
    • 0.0.0.0/0
      • The prefix-length is /0, therefore the network mask is 0.0.0.0.
      • You may recognize this as the default route.
      • If no other entry in the routing table matches, then the packets will get routed out this default route.
      • The prefix-length is zero so it is the shortest prefix-length in the routing table and therefore the last option to be considered as a route.
    • 172.16.0.0/16
      • The prefix-length is /16, which means the network mask is 255.255.0.0
      • This is a summary route for the four 172.16.x.x networks on this device
      • Because it's a summary route it doesn't have a routing code or next hop.  It's helpful information describing how the routing table is organized.
    • 172.16.1.0/24
      • The prefix-length is /24 and the network mask is 255.255.255.0
    • 172.16.1.1/32
      • The prefix-length is /32 and the network mask is 255.255.255.255
      • This is the most specific prefix-length possible in IPv4
      • Being more specific than the /24 directly above, this route will be preferred for this specific IP address
    • Your turn!  What are the prefix-lengths below?  For extra credit, what are the network masks?
    • 172.16.2.0/24
    • 172.16.3.0/24
    • 172.16.4.0/28
    • 192.168.0.0/16
    • 209.165.200.0/24
    • 209.165.200.224/30
    • 209.165.200.224/32
    • 209.165.200.225/32
    • 209.165.200.228/30
    • 209.165.200.232/30
    • 209.165.200.233/32
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