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Subnetting and Supernetting

This document explores subnetting and supernetting (CIDR) in the context of network routing. It highlights the inefficiencies of hierarchical addressing, particularly with classful addressing, and discusses how subnetting can enhance scalability. The paper analyzes the structure of Autonomous Systems and discusses their independent administration, varying routing protocols, and link metrics. The implications of subnet masks, routing tables, and the forwarding process in routers are examined. It culminates with an overview of CIDR, its benefits, and efficiency concerns.

jthornhill
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Subnetting and Supernetting

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  1. Subnetting and Supernetting CS 332

  2. How to Make Routing Scale • Flat versus Hierarchical Addresses • Inefficient use of Hierarchical Address Space • class C with 2 hosts (2/255 = 0.78% efficient) • class B with 256 hosts (256/65535 = 0.39% efficient) • Demand for Class B the problem. So why not just assign 2 class C’s for a 50% efficiency rate? • Still Too Many Networks • routing tables do not scale • route propagation protocols do not scale CS 332

  3. Internet Structure • Autonomous system (AS) • Administered Independently of other ASs • Want to be able to control various ways in which network is configured, used, etc. • Select their own intradomain routing protocol • Perhaps select own link metrics, etc. • Advantageous because it provides finer hierarchy • Good for scalability CS 332

  4. Network number Host number Class B address 111111111111111111111111 00000000 Subnet mask (255.255.255.0) Network number Subnet ID Host ID Subnetted address Subnetting • Add another level to address/routing hierarchy: subnet • Subnet masks define variable partition of host part • Subnets visible only within site CS 332

  5. Subnet Mask • Written in dotted quad notation (like IP addresses) • Exactly one mask per subnet (all hosts on given subnet have same subnet mask) • Subnet number of host (or of subnet) = bitwise AND of subnet mask and IP address 11111111 11111111 11111111 10000000 10000000 01100000 00100010 00001111 10000000 01100000 00100010 00000000 CS 332

  6. Subnetting (cont) • To send IP packet: • Host performs bitwise AND of its subnet mask with destination IP address • If result is same subnet number as sending host, then destination is on same subnet, so forward directly (Note: Arp unaffected) • Else send packet to a router to be forwarded to another subnet • New routing table entries: <SubnetNumber, SubnetMask, NextHop> replaces <NetworkNumber, NextHop> CS 332

  7. Subnet mask: 255.255.255.128 Subnet number: 128.96.34.0 128.96.34.15 128.96.34.1 H1 R1 Subnet mask: 255.255.255.128 128.96.34.130 Subnet number: 128.96.34.128 128.96.34.139 128.96.34.129 H2 R2 H3 128.96.33.1 128.96.33.14 Subnet mask: 255.255.255.0 Subnet number: 128.96.33.0 Subnet Example Forwarding table at router R1 Subnet Number Subnet Mask Next Hop 128.96.34.0 255.255.255.128 interface 0 128.96.34.128 255.255.255.128 interface 1 128.96.33.0 255.255.255.0 R2 CS 332

  8. Forwarding Algorithm D = destination IP address for each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to D else deliver datagram to NextHop • Use a default router if nothing matches • Not necessary for all 1s in subnet mask to be contiguous • Can put multiple subnets on one physical network(?!) • Subnets not visible from the rest of the Internet CS 332

  9. The Key • It’s important to remember that both subnetting and supernetting are attempts to help make routing scale • Even for an AS like U of R, subnetting can help shrink routing tables, though this isn’t really a serious issue here • Supernetting is really intended to make Internet routing scale – it benefits primarily the internet service providers and backbone routers, where the real scale problems exist. CS 332

  10. 3185* 319* 52* 31* 51* 5* backbone 317* 3172* 534* 7* 3174* 73* 76* 748* 317483* 317482* Supernetting (CIDR) • What we’re shooting for: CS 332

  11. Supernetting (CIDR) • Called CIDR: Classless Inter-Domain Routing • Assign block of contiguous network numbers to nearby networks (in same AS or using same ISP) • Aggregates routes: single entry for many networks • E.g. Class B addresses 192.4.16-192.4.31 have same top 20 bits, so a single 20 bit network address gets packets to correct AS. • Restrict block sizes to powers of 2 • Represent network numbers with(length, value)pair • All routers must understand CIDR addressing CS 332

  12. A Caveat • There is a serious issue regarding the efficiency of CIDR. Can you determine what it is? • Do you know what a Trie is? • I’m putting together a handout that will explain this, (And I may have you read the original paper…) CS 332

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