Subnetting and Supernetting

Network Prefix and Host number IP address consists of a network prefix and a host number Prefix notation: 128.100.__.__/16 Notation with netmask: 128.100.__.__ 255.255.0.0

Actual Network Prefix IP address consists of a network prefix and a host number Prefix notation: 128.100.__.__/__ Notation with netmask: 128.100.__.__ 255.255.__.__

Faculty of Engineering Subnetting UofT Network Scenario: Organization has a large network prefix and wants to create smaller networks. Subnetting: Use a portion of the host name to identify a smaller network (“subnetwork”, “subnet”). Each subnet becomes a separate network 128.100.0.0/16 Faculty of Engineering Faculty of A&S 128.100.11.0/24 128.100.58.0/24 Library 128.100.136.0/24

Basic Idea of Subnetting Split the host number portion of an IP address into a subnet number and a (smaller) host number. Result is a 3-layer hierarchy Then: Subnets can be freely assigned within the organization Internally, subnets are treated as separate networks Subnet structure is not visible outside the organization network prefix host number network prefix subnet number host number extended network prefix

Subnetmask Start of the host number is indicated by prefix or netmask notation The netmask of the extended network prefix is called subnetmask (and the nework is called subnet)

Example Extended network prefix: Address of host: IP address of host: 128.100.11.60 (original) Network prefix: 128.100.0.0/16 Subnet number (with length 8 bit) : 11 Extended network prefix: Prefix notation: 128.100.11.0/24 Netmask notation: 128.100.11.0, 255.255.255.0 Address of host: Prefix notation: 128.100.11.60/24 Netmask notation: 128.100.11.60, 255.255.255.0

Advantages of Subnetting With subnetting, IP addresses use a 3-layer hierarchy: Network Subnet Host Reduces router complexity. Since external routers do not need to know about subnetting, the complexity of routing tables at external routers is reduced. Flexibility: Length of the subnet mask need not be identical on all subnetworks.

Creating and Managing Subnets Use of subnetting or length of the subnetmask is decided by the network administrator Subnets are created by setting the subnetmask of an IP interface Important: Subnetmasks must be set consistently

No Subnetting All hosts think that the other hosts are on the same network

With Subnetting Hosts with same extended network prefix belong to the same network

With Subnetting Different subnetmasks lead to different views of the scope of the network

Subnetting and Supernetting Subnets are created by extending the network prefix Supernetting: Multiple prefixes can be summarized with a single prefix, by reducing the network prefix: 128.143.0.0/16 128.142.0.0/16 128.142.0.0/15 If neighboring networks have similar address blocks, supernetting reduces the size of routing tables Route Aggregation: Routing table entries can be reduced, when prefixes can be collapsed and networks have the same outgoing interface

CIDR and Hierarchical IP address allocation Supernetting helps with reducing size of routing tables: Backbone ISPs obtain blocks of IP addresses and allocate portions of their address blocks to their customers Customers can allocate a portion of their address block to their own customers Note: This leads to a hierarchical organization of networks

Hierarchical Structure of IP Networks Company Y : ISP X owns: 65.10/16 Rest of Internet ISP Z : Organization A: Organization B:

Hierarchical Structure of IP Networks Company Y : 65.11 /16 ISP X owns: 65.10/15 Obtained from ISP X Obtained from ISP X Rest of Internet ISP Z : 65.10/16 Obtained from ARIN Obtained from ISP Z Obtained from ISP Z Organization A: 65.10.11/24 Organization B: 65.10.12/24

Hierarchical Structure of IP Networks Company Y : 65.11/16 ISP X owns: 65.10/15 Rest of Internet Routing table entry for 65.11/16 and 65.10/16 (no entry needed for 65.10.11/24 and 65.10.12/24) ISP Z : 65.10/16 Only one routing entry for 65.10/15 No entries needed for network addresses of Company Y, ISP Z, or Organizations A, B. Organization A: 65.10.11/24 Organization B: 65.10.12/24

Drawback of Hierarchical IP Networks IP address assignment depends on service provider Company Y : 65.11/16 ISP X owns: 65.10/15 Rest of Internet ISP Z : 65.10/16 Must change IP addresses when changing service provider Organization A: 65.10.11/24 Organization B: 65.11.12/24 65.10.12/24

Routing Table Aggregation Routing table entries that can be summarized and that have same outgoing interface can be collapsed. Network Prefix Interface 128.100.0.0/16 interface #5 128.101.0.0/16 interface #4 64.10.0.0/20 interface #2 64.10.16.0/20 100.40.128.0/21 interface #3 100.40.136.0/21 100.40.144.0/21 100.40.152.0/21 Routing Table

Routing Table Aggregation Routing table entries that can be summarized and that have same outgoing interface can be collapsed. Network Prefix Interface 128.100.0.0/16 interface #5 128.101.0.0/16 interface #4 64.10.0.0/20 interface #2 64.10.16.0/20 100.40.128.0/21 interface #3 100.40.136.0/21 100.40.144.0/21 100.40.152.0/21 Network Prefix Interface 128.100.0.0/16 interface #5 128.101.0.0/16 interface #4 64.10.0.0/19 interface #2 100.40.128.0/19 interface #3 Routing Table