1. Subnetting Surasak Sanguanpong nguan@ku.ac.th
Last updated: 27 June 2002

2. Topics The Basics of Subnetting Subnet Mask
Computing subnets and hosts
Subnet Routing
Creating a Subnet
Example of Subnetting

3. Addressing without Subnets
A class B “Flat Network”, more than hosts
How to manage?
Performance?

4. Addressing with Subnets
Network administrators sometimes need to divide networks, especially large ones, into smaller networks. These smaller divisions are called subnetworks and provide addressing flexibility. Most of the time subnetworks are simply referred to as subnets.
Similar to the host number portion of Class A, Class B, and Class C addresses, subnet addresses are assigned locally, usually by the network administrator. Also, like other IP addresses , each subnet address is unique.
A class B “subdivided network”, smaller groups with routers

5. Subnetwork benefits Subnetwork
Smaller networks
are easier to manage and troubleshoot
Increase the network manager's control over the address space
Overall traffic is reduced, performance
may improve
Subnetwork
A primary reason for using subnets is to reduce the size of a broadcast domain. Broadcasts are sent to all hosts on a network or subnetwork. When broadcast traffic begins to consume too much of the available bandwidth, network administrators may choose to reduce the size of the broadcast domain.
Subdivide on IP network number is an important initial task of network managers

6. Subnet Address Before Subnetting Network ID Host ID After Subnetting
Subnet ID
Host ID
To create subnets, you must extend the routing portion of the address. The Internet knows your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number). The subnet field will become additional routing bits, so that the routers within your organization can recognize different locations, or subnets, within the whole network
Subnet addresses include the Class A, Class B, or Class C network portion, plus a subnet field and a host field. The subnet field and the host field are created from the original host portion for the entire network. The ability to decide how to divide the original host portion into the new subnet and host fields provides addressing flexibility for the network administrator. To create a subnet address, a network administrator borrows bits from the original host portion and designates them as the subnet field.
A subnet address is created by borrowing bit from the Host ID and designated it as a Subnet ID field

7. Define physical subnetwork Define individual hosts
How to assign subnet
Each class can have different size of subnet field
Define physical subnetwork
Define individual hosts
Network
Subnet
Host
choose
appropriate size
Figures and illustrate the hierarchical nature of subnet addresses. 
To create a subnet address, a network administrator borrows bits from the host field and designates them as the subnet field. The minimum number of bits that can be borrowed is 2. If you were to borrow only 1 bit, to create a subnet, then you would only have a network number - the .0 network - and the broadcast number - the .1 network. The maximum number of bits that can be borrowed can be any number that leaves at least 2 bits remaining, for the host number. In this example of a Class C IP Address, bits from the host field for the subnet field have been borrowed.  
Class Size of Default Host Field Max Number of Subnet Bits A B C
Previous standards did not allow for the use of subnets obtained by borrowing 1 bit  (with only 1 subnet bit, the subnet field can only have two values: subnet 0 is part of the network address, and subnet 1 would be part of the network broadcast address) – although many devices can now support subnets obtained by borrowing 1 bit, it is still common practice to avoid doing this to insure compatibility with legacy devices; for our purposes here, you will always borrow at least 2 bits.
2 Similarly, a 1 bit host field would allow only for host 0, which is part of the network address, and host 1, which is part of the broadcast address, leaving 0 valid host addresses
Class A : 2 to 22 bits
Class C : 2 to 6 bits
Class B : 2 to 14 bits

8. Subnet Example
Class B address such as might use its third byte to identify subnet
Subnet
Network Address
Address Range #1
172.
16. 1. #2
172.
16. 2. #3
172.
16. 3.
#254
172.
16.
254.

9. Subnet mask
subnet mask is a 32 bit number, use to identify a subnet
Example : A class B network with 24 bits mask
Network ID
Subnet ID
Host ID
Set the bit covering the network and subnet ID to 1 1
255. 0.
zero bit are used to mask out the host number
resulting the network address 2
The subnet mask (formal term: extended network prefix), is not an address, but determines which part of an IP address is the network field and which part is the host field. A subnet mask is 32 bits long and has 4 octets, just like an IP address.  To determine the subnet mask for a particular subnetwork IP address follow these steps. (1) Express the subnetwork IP address in binary form. (2) Replace the network and subnet portion of the address with all 1s. (3) Replace the host portion of the address with all 0s. (4) As the last step convert the binary expression back to dotted-decimal notation.
Note: The extended network prefix includes the class A, B, or C network number, plus the subnet field (or subnet number) that is being used to extend the routing information (which is otherwise just the network number).
subnet mask=

10. Masking
& 1 1 1
The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet.
In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address.
A “bitwise-and” between IP address and subnet mask yields a network address.
Note that zeros bit are used to mask out the host number
resulting the network address

11. Subnet mask in Prefix format
The number of routing bits (network and subnet bits) in each subnet mask can also be indicated by the "/n " format.  /8
/18
/24
/28 =
/24

12. Subnet routing
Traffic is routed to a host by looking “bit-wise and” results
if dest_ip_addr & subnet_mask = = my_ip_addr & subnet_mask
send pkt on local network %dest ip addr is on the same subnet
else
send pkt to router %dest ip addr is on diff subnet
The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet. In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address.

13. Routing Hosts and routers perform logical AND to send packets 1 To 1
/24
/24 2 3
/24
/24
The lowest numbered address in an IP network is the network address (the network number plus 0 in the entire host field). This also applies to a subnet: the lowest numbered address is the address of the subnet.
In order to route a data packet, the router must first determine the destination network/subnet address by performing a logical AND using the destination host's IP address and the subnet mask. The result will be the network/subnet address.
In the Figure, the router has received a packet for host it uses the AND operation to learn that this packet should be routed to subnet The process of ANDing is explained in Lab
has a packet for and determine that it is on other subnetwork
The packet is sent to the router
The router performs a subnet masking and sends the packet to the destination network

14. Subnet interpretation
IP Address
subnet mask
Interpretation
host 15.2 on subnet
host 3 on subnet
host 4th on subnet
host 71 on subnet
host 2nd on subnet

15. Default Subnet mask
A default subnet mask : a subnet mask with no subnetting
Class A
Class B
By default, if you borrow no bits, the subnet mask for a Class B network would be , which is the dotted decimal equivalent of 1s in the 16 bits corresponding to the Class B network number.
Class C

16. Range of bit A default subnet mask : a subnet mask with no subnettingIP
172 16
Default subnet
255
255
New subnet
To create subnets, you must extend the routing portion of the address. The Internet knows your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24 routing bits (the network number). The subnet field will become additional routing bits, so that the routers within your organization can recognize different locations, or subnets, within the whole network.
By default, if you borrow no bits, the subnet mask for a Class B network would be , which is the dotted decimal equivalent of 1s in the 16 bits corresponding to the Class B network number.
If 8 bits were to be borrowed for the subnet field, the subnet mask would include 8 additional 1 bits, and would become
For example, if the subnet mask were associated with the Class B address (8 bits borrowed for subnetting), the router would know to route this packet to subnet rather than to just network
255
255
255
Define a subnet mask by extending the network portion to the right, 8 bits in this example

17. Computing subnet mask Decimal equivalents of bit patterns Binary mask
128
192
224
240
248
252
254
255
Binary mask
Octet value
Whenever you borrow bits from the host field, it is important to note the number of  additional subnets that are being created each time you borrow one more bit. You have already learned that you cannot borrow only 1 bit; the fewest you may borrow is 2 bits. Borrowing 2 bits creates four possible subnets (22) (but you must always remember that there are two reserved/unusable subnets). Each time you borrow another bit from the host field, the number of subnets created increases by a power of 2.
The eight possible subnets that are created by borrowing 3 bits is equal to 23 (2 x 2 x 2). The sixteen possible subnets created by borrowing 4 bits is equal to 24 (2 x 2 x 2 x 2). From these examples, it is easy to see that each time you borrow another bit from the host field, the number of possible subnets doubles.

18. Compute Net and host
How many subnet and host are there with /24
255. 0.
Network ID
Subnet ID
Host ID
Each time you borrow 1 bit from a host field, there is 1 less bit remaining in the field that can be used for host numbers. Specifically, each time you borrow another bit from the host field, the number of host addresses that you can assign decreases by a power of 2 (gets cut in half). To help you understand how this works, use a Class C network address as an example. If there is no subnet mask, all 8 bits in the last octet are used for the host field. Therefore, there are 256 (28) possible addresses available to assign to hosts (254 usable addresses, after you subtract the 2 you know you can't use). Now, imagine that this Class C network is divided into subnets. If you borrow 2 bits from the default 8 bit host field, the host field decreases in size to 6 bits. If you write out all of the possible combinations of 0s and 1s that could occur in the remaining 6 bits, you would discover that the total number of possible hosts that could be assigned in each subnet would be reduced to 64 (26). The number of usable host numbers would be reduced to 62.
In the same Class C network, if you borrow 3 bits, the size of the host field decreases to 5 bits and the total number of hosts that you could assign to each subnet would be reduced to 32 (25). The number of usable host numbers would be reduced to 30. 
The number of possible host addresses that can be assigned to a subnet is related to the number of subnets that have been created. In a Class C network, for example, if a subnet mask of has been applied, then 3 bits (224 = ) would have been borrowed from the host field. The useable subnets created are 6 (8 minus 2), each having 30 (32 minus 2) useable host addresses.
8 bit subnet ID = 28=256 => 254 subnets
8 bit host ID = 28=256 => 254 hosts per subnet

19. Network and Host relationship
Sample class C
Number of subnet bits
Number of subnets created
Number of hosts per subnet
Total number of hosts
Percents
used 2 62
124
49% 3 6 30
180
71% 4 14
196
77% 5
One of the decisions that you must make whenever you create subnets is to determine the optimal number of subnets and hosts (Note: The number of subnets required in turn determines the number of hosts available. For example, if you borrow 3 bits with a Class C network, only 5 bits remain for hosts). Lab Activity 
This lab focuses on a Class C network with three subnets and using a Custom Subnet Mask. It will help you develop a better understanding of IP subnet masks.
You have already learned that you cannot use the first and last subnet. You also cannot use the first and last address within each subnet - one is the broadcast address of that subnet, and the other is part of the network address. When you create subnets, you lose quite a few potential addresses. For this reason, network administrators must pay close attention to the percentage of addresses that they lose by creating subnets.
Example: If you borrow 2 bits with a Class C network, you create 4 subnets, each with 64 hosts. Only 2 of the subnets are usable and only 62 hosts are usable per subnet, leaving 124 usable hosts out of 254 that were possible before you chose to use subnets. This means you are losing 51% of your addresses.
Imagine, this time, that you borrow 3 bits. You now have 8 subnets, of which only 6 are usable, with 30 usable hosts per subnet. This gives you a total of 180 usable hosts, down from 254, but now you are losing only 29% of your addresses. Whenever you create subnets, you need to take into consideration future network growth and the percentage of addresses that you would lose by creating subnets.

20. Subnetting Special Addresses
Reserved addresses that are not allowed to be assigned to any node
NetID
HostID
Purpose
any
All 0s
Subnetwork Address
Example: /24
Subnetwork
any
All 1s
Subnet-directed Broadcast
Example: /24
Directed broadcast of the subnetwork

21. Subnet Net Block Diagram
No subnetting
2 bits
Block diagram subnetting class C
3 bits
4 bits
Network Address
Broadcast Address

22. Contiguous and Noncontiguous mask
no intermedite 0 gaps in the subnet mask
Contiguous subnet mask
intermedite 0 gaps in the subnet mask
Noncontiguous subnet mask
Noncontiguous leads to complex subnetting and routing
It is strongly recommend to use contiguous subnet mask

23. Subnet Class A Example subnet mask Interpretation 255.0.0.0
1 network with hosts (default subnet)
254 subnets each with hosts
510 subnets each with hosts
1022 subnets each with hosts
65534 subnets each with 254 hosts

24. Example : Class A Subnet Address Table
IP Address : /16
SubnetID all 0s #1 #2
Network Address
Broadcast Address
#254
SubnetID all 1s

25. Class A Subnet with router
/16
254 subnets each
with hosts to #1 to #2 #3 to
#254 to

26. Subnet Class B Example subnet mask Interpretation 255.255.0.0
1 network with hosts (default subnet)
2 subnets each with hosts
62 subnets each with 1022 hosts
254 subnets each with 254 hosts
16382 subnets each with 2 hosts

27. Example : Class B Subnet Address Table
IP Address : /24
SubnetID all 0s #1 #2
Network Address
Broadcast Address
#254
SubnetID all 1s

28. Class B Subnet with router to
/24
254 subnets each
with hosts #1 to #2 #3 to to
#254

29. Subnet Class C Example subnet mask Interpretation 255.255.255.0
1 network with 254 hosts (default subnet)
2 subnets each with 62 host
6 subnets each with 30 hosts
14 subnets each with 14 hosts
62 subnets each with 2 hosts

30. Example : Class C Subnet Address Table
IP Address : /27
SubnetID all 0s #1 #2
Network Address
Broadcast Address #6
SubnetID all 1s

31. Class C Subnet with router
/27
6 subnets each with 30 hosts to #1 to #2 #3 to to #6

32. Subnet Exercise (1)
Given IP address , find out the following to yield not more than 256 hosts per subnet
net mask= ??
start net id =??
end net id=??
#of subnet =??

33. Subnet Exercise (2)
Given IP address , find out the following to yield not more than 32 hosts per subnet
net mask= ??
start net id =??
end net id=??
#of subnet =??

34. Type of Subnetting Static Subnetting Variable Lengh Subnetting
all subnets in the subnetted network use the same subnet mask
pros: simply to implement, easy to maintain
cons: wasted address space (consider a network of 4 hosts with wastes 250 IP)
the subnets may use different subnet masks
pros: utilize address spaces
cons: required well-management
Variable Lengh Subnetting

35. Problem of Static subnetting
/27
used 20 hosts,
waste 10 hosts
/27
used 20 hosts,
waste 10 hosts
/27
used 25 hosts,
waste 5 hosts
Inefficient allocation of the address space
/27
used 25 hosts,
waste 5 hosts
/27
used 10 hosts,
waste 20 hosts
/27
used 10 hosts,
waste 20 hosts

36. Variable-Length Subnetting
/27
used 20 hosts, waste 10 hosts
General Idea of VLSM
A small subnet with only a few hosts needs a subnet mask that accommodate only few hosts
A subnet with many hosts need a subnet mask to accommodate the large number of hosts
/27
used 20 hosts, waste 10 hosts
/27
used 25 hosts, waste 5 hosts
/27
used 25 hosts, waste 5 hosts
/28
used 10 hosts, waste 4 hosts
/28
unused subnet
Available 14 hosts
/28
used 10 hosts, waste 4 hosts
/28
unused subnet
Available 14 hosts

37. VLSM - An Example three different VLSM of 172.16.0.0 CPC RDI
point-to-point link
CPE