1. CS4470 Computer Networking Protocols
7/23/2018
CS Computer Networking Protocols
10. Subnetting
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. 7/23/2018
Subnetting
Subnetting is the division of a single network into smaller subnets.
Each subnet functions as if it were an independent network
As far as entries in the routing table are concerned
All subnets of a given network number use the same network-prefix but different subnet numbers.
10. Subnetting CS4470_F17

3. Why are subnets created?
Provides greater organization of large networks.
Remember: a Class A network can have up to 16 million hosts.
It allows you to have additional networks without applying for more IP addresses.
Decreases broadcast traffic by reducing broadcast domain size.
Broadcasts are not forwarded out of a subnet.
10. Subnetting CS4470_F17

4. How to subnet a network?
Borrow bits from the host field of an IP address.
All the subnets have the same network prefix
Subnets are only visible to routers inside the network
Netid subnetid hostid 16 24 31
10. Subnetting CS4470_F17

5. Subnet a network: steps
Determine the number of subnets
How many bits to be borrowed?
For each subnet, determine
The network address
subnet bits cannot be all on or off at the same time
The host address range
The broadcast address
Determine the number of required host per subnet:
One for each host
One for each router interface
10. Subnetting CS4470_F17

6. Subnetting example 1:
You are assigned a network address /23. You want to create 2 subnets.
What are the network addresses and broadcast addresses of the 2 subnets?
What are the address range in the 2 subnets?
10. Subnetting CS4470_F17

7. Subnetting example 1: 11010110 01100001 11111110 0 000 0000 Subnet 1:23
Subnet bits
    
Subnet 1:
Network address:      /25
Broadcast address:      /25
Address range:      /25
     /25
126 IP addresses
10. Subnetting CS4470_F17

8. Subnetting example 1: Subnet 2:
Network address:      /25
Broadcast address:      /25
Address range:      /25
     /25
126 IP addresses
10. Subnetting CS4470_F17

9. Subnetting example 2:
You are assigned a network address /23. If you assign 3 bits to the subnets.
How many subnets can be created
What are the network addresses and broadcast addresses of the subnets?
What are the address range of the subnets?
10. Subnetting CS4470_F17

10. Subnetting example 2: 23
Subnet bits
    
The number of subnets: 23-2=6
Subnet 1:
Network address:      /26
Broadcast address:      /26
Address range:      /26
     /26
26-2=62 IP addresses
10. Subnetting CS4470_F17

11. Subnetting example 2: Subnet 2:
Network address:      /26
Broadcast address:     
Address range:     
    
26-2=62 IP addresses
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12. Subnetting example 2: Subnet 3:
Network address:      /26
Broadcast address:     
Address range:     
    
26-2=62 IP addresses
10. Subnetting CS4470_F17

13. Subnetting example 2: Subnet 4:
Network address:      /26
Broadcast address:     
Address range:     
    
26-2=62 IP addresses
10. Subnetting CS4470_F17

14. Subnetting example 2: Subnet 5:
Network address:      /26
Broadcast address:     
Address range:     
    
26-2=62 IP addresses
10. Subnetting CS4470_F17

15. Subnetting example 2: Subnet 6:
Network address:      /26
Broadcast address:     
Address range:     
    
26-2=62 IP addresses
10. Subnetting CS4470_F17

16. Subnetting example 3
A large number of consecutive IP address are available starting at /16. Suppose that six  organizations, A,B,C, D and E, F, request 2000, 2000, 4000, 4000, 8000 and 8000 addresses, respectively, and in that order. For each of  organizations, give the network address, broadcast address, the first IP address assigned and the last IP address assigned in the w.x.y.z/s notation.
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17. Subnetting example 3
A needs 11 bits for host id, which have 2048 IP addresses available.
Network A          
                     Network Address: /21
                   Broadcast Address: /21
                     2046 IP Addresses: /21 – /21
B needs 11 bits for host id, which have 2048 IP addresses available.
Network B         
                  Network Address:
                 Broadcast Address: /21
                 2046 IP Addresses: /21 – /21
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18. Subnetting example 3
C needs 12 bits for host id, which have 4096 IP addresses available.
Network C         
                   Network Address: /20
                   Broadcast Address: /20
                   4094 IP Addresses: /20 – /20
D needs 12 bits for host id, which have 4096 IP addresses available.
Network D         
                   Network Address: /20
                   Broadcast Address: /20
                   4094 IP Addresses: /20 – /20
10. Subnetting CS4470_F17

19. Subnetting example 3
E needs 13 bits for host id, which have 8192 IP addresses available.
Network E         
                   Network Address: /19
                   Broadcast Address: /19
                   8190 IP Addresses: /19 – /19
F needs 13 bits for host id, which have 8192 IP addresses available.
Network F         
                   Network Address: /19
                   Broadcast Address: /19
                   8190 IP Addresses: /19 – /19
10. Subnetting CS4470_F17