1. Subnetting
and
Supernetting

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

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

4. 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
/16
Faculty of Engineering
Faculty of A&S
/24
/24
Library
/24

5. 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

6. 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)

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

8. 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.

9. 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

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

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

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

13. 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:
/16
/ /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

14. 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

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

16. 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:
/24
Organization B:
/24

17. 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 /24 and /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:
/24
Organization B:
/24

18. 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:
/24
Organization B:
/24
/24

19. Routing Table Aggregation
Routing table entries that can be summarized and that have same outgoing interface can be collapsed.
Network Prefix
Interface
/16
interface #5
/16
interface #4
/20
interface #2
/20
/21
interface #3
/21
/21
/21
Routing Table

20. Routing Table Aggregation
Routing table entries that can be summarized and that have same outgoing interface can be collapsed.
Network Prefix
Interface
/16
interface #5
/16
interface #4
/20
interface #2
/20
/21
interface #3
/21
/21
/21
Network Prefix
Interface
/16
interface #5
/16
interface #4
/19
interface #2
/19
interface #3
Routing Table