[Network week2] Forwarding, Routing table, Address Aggregation, Longest Prefix Matching

makeitcloud·2020년 9월 24일

Computer Science

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  • Router at Home

    : Connect mobiles and notebooks to Internet
    : Implement NAT and DHCP functions

  • Router in backbone

    : 35 interface card
    : 35 different network addresses are available

  • What does a router do?

    : Connect two or more networks
    Has two or more network interfaces and IP addresses

  • Functions in Network Layer

    -- Main objective of network layer
    : Transport packets from source to destination
    -- Two mechanisms are used in network layer
    : Forwarding and routing

  • Forwarding and Routing

  1. Forwarding

: Mechanism to determine an interface each packet should be sent to reach its destination
: Move packets from router's input to appropriate router output by looking up the routing table

  1. Routing : control plane

: Mechanism that distributes to all routers information to build their routing tables
: Each router computes paths packets will follow
: Routers talking amonst themselves
: Based on sharing information individual router creates a routing table


  • Efficient look-up to a routing table
  • A format and size of a routing table matter

Route vs. Next-hop (win)

: Not specify an entire route (path) to destination in the routing table
--> Only next-hop host toward destination

Host-specific vs Network-specific (win)

: Too many hosts in Internet to remember
--> A, B, C and D can be summarized in network N2

Direct against Indirect Delivery

  • Direct delivery for hosts inside networks
    : Using host address
  • Indirect delivery for hosts outside networks
    : Using network address

Default Method

  • Further decrease the size of routing table by default method
    : Specify all known networks in the table and rest of the networks is included in a default

Common Fields in Routing Table

  • Forwarding based on destination
  • Network-specific routing table
    : Forwarding table is searched based on network address
    : Destination address in the packets gives no clue about the network address
  • When a host has a packet to send or a router has received a packet to be forwarded
    : It looks a routing(forwarding) table to find the next-hop IP address to deliver the packet to
    : compare destination IP address with each entry in the routing table to find a match

Sample Routing Table in Linux


Address Aggregation

  • For classful, a single entry for each site in the table
  • For classless, it is likely that the number of forwarding table entries will increase
    : Division of whole address space into blocks
    : Each blocks needs to be in the table

Longest Prefix Matching

  • Org. 4 cannot be connected to R1 for some reasons
  • A packet arrives at R2 with DST


-> Routing Table for R1

Hierarchical Routing

  • A regional ISP is granted 16,384 addresses starting from
    : The regional ISP has decided to divide this block into four subblocks, each with 4096 addresses
    : 01111000 00001110 01000000 00000000
    : Three of these subblocks are assigned to three local ISPs.
    : Second subblock is reserved for future use


  • The first local ISP has divided its assigned subblock into 8 smaller blocks and assigned each to a small ISP
    : 4096= 2^12 -> 2^12/2^3 = 512
    : 01111000 00001110 01000000 00000000 /
    : 01111000 00001110 01001000 00000000 /
  • The second local ISP has divided its block into 4 blocks and has assigned the addresses to four large organizations
  • The third local ISP has divided its block into 16 blocks and assigned each block to a small organization. Each small organization has 256 addresses, and the mask is /24.
  • There is a sense of hierarchy in this configuration
    : All routers in the Internet send a packet with destination address to to the border router in the regional ISP

Geographical Routing

  • Divide entire address psace into a few large blcks
    : Assign a block to America, a block to Europe and so on
  • Router of ISPs outside of Europe will have only single entry for packets to Euripe in their forwarding table

Business & Software

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