Can you expain summarization?

Raymond MasonRaymond Mason Posts: 74Member ■■□□□□□□□□
Hello! I have Todd Lammles book for study material. I understand how to subnet and I understand VSLM. What I am having trouble with is summarization.

I understand that its used to just to summarize a bunch of networks to ease router memory. I get the gist of what its meant to do; but the book words it in a way where I don't understand how its done.

For example: through network

He says there is exactly 16 class 16 Class C networks. I don't quiet get where he got 16 networks from; he doesn't give a notation or subnet mask. Maybe you guys can word this different? Thanks again!


  • PCTechLincPCTechLinc CISSP, CHFI, CEH, MCSA Server 2008, Project+, Security+ce, Server+, Network+, A+ King City, CAPosts: 621Member ■■■■■□□□□□ and both have a default subnet mask of That means that past the last octect (.0), every number change is a different network. So, 16.0 - 31.0 are 16 separate networks.

    If you change the subnet mask to, then that will summarize all those 16 networks into one block, and when you look at the router graphically, you'll see that the router knows how to forward ANY address from ANY of those networks out of one interface. Hence, why summarization "compresses" the routing table so efficiently.

    Hope that makes sense!
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  • nice343nice343 Posts: 391Member
    I hope the attached table gives you some clarification

    It shows the number of /24 each subnet mask will have.

    The range for a /20 is x.y.0.1 - x.y.15.254 (16 /24 subnets including the 0)

    Based on your example through network

    If we look at the 3rd octet
    16 + 15 = 31
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  • pinkiaiiipinkiaiii Posts: 216Member
    I ran into simillar issue but with wildcard masks included in ospf or eigrp-since seems for some networks when you summarize you can use like ip,in other cases you need to use specifically or am i crossing between the two in the wrong way where if using full address and wildcard mask is just for single ip, where if using for summarized route ?
  • james43026james43026 Posts: 303Member
    I'm going to take a different approach to answering this. Route summarization / aggregation is also called supernetting. I'll assume you understand subnetting, and the fact that under all circumstances, when you subnet, you are borrowing host bits, in order to increase the number of network bits. Supernetting is the opposite, you are borrowing network bits in order to increase the number of host bits, this essentially takes several smaller networks, and aggregates them into one larger network. Let me know if you need an example.
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