Calculation and Computing of Subnets
Networking II Calculation and
Computing of
Subnets
ENGR. RUEL G. GRAFIA
SORSOGON STATE UNIVERSIRY
Assistant Professor
ENGR. RUEL G. GRAFIA 1
Calculation and Computing of Subnets
Part II. IPv4 Subnetting
IPv4 allows for a variation of the network and host segments of an IP address, known
as subnetting, can be used to physically and logically design a network. For example, an
organization can have a single internet network address (NETID) that is known to users outside
the organization, but configure its internal network into different departmental subnets.
Subnetwork addresses enhance local routing capabilities, while reducing the number of network
addresses required.
How to Create Subnets
To create a subnet, we’ll start by fulfilling these three steps:
1. Determine the number of required network IDs:
o One for each LAN subnet
o One for each wide area network connection
2. Determine the number of required host IDs per subnet:
o One for each TCP/IP host
o One for each router interface
3. Based on the previous requirements, create the following:
o A unique subnet mask for your entire network
o A unique subnet ID for each physical segment
o A range of host IDs for each subnet
Subnetting Class B Addresses
There are many different ways to subnet a network. The right way is the way that works best for
you. In a Class B address, only 16 bits are available for defining the hosts. Remember that subnet
bits start at the left and move to the right, without skipping bits. This means that the only Class
B subnet masks can be the following:
Mask (slash notation or CIDR) Mask (slash notation or CIDR)
255.255.0.0 (/16) 255.255.255.0 (/24)
255.255.128.0 (/17) 255.255.255.128 (/25)
255.255.192.0 (/18) 255.255.255.192 (/26)
255.255.224.0 (/19) 255.255.255.224 (/27)
255.255.240.0 (/20) 255.255.255.240 (/28)
255.255.248.0 (/21) 255.255.255.248 (/29)
255.255.252.0 (/22) 255.255.255.252 (/30)
255.255.254.0 (/23)
ENGR. RUEL G. GRAFIA 2
Calculation and Computing of Subnets
Subnetting Practice Examples: Class B Addresses
Practice Example #1:
Given: 172.16.0.0 = Network address
255.255.128.0 = Subnet mask
Required: Formula: (2n = subnet, 2n – 2 = hosts per subnet)
§ Number of subnet ?
§ Hosts per subnet ?
§ Valid Subnet or custom subnet mask ?
§ Hosts range (first host – last host) ?
§ Broadcast address ?
Solution: You need to borrow 1-bit of 1’s
§ Subnets? 21 = 2 (same amount as Class C).
§ Hosts? 215 – 2 = 32,766 (7 bits in the third octet, and 8 in the fourth).
§ Valid subnets? 256 – 128 = 128. 0, 128. Remember that subnetting is performed in the
third octet, so the subnet numbers are really 0.0 and 128.0
These are the exact numbers we used with Class C; we use them in the third octet and
add a 0 in the fourth octet for the network address.
Alternative solution to find the valid subnet: (by applying the increment value = 128)
§ Broadcast address for each subnet ? 172.16.127.255, 172.16.255.255
§ Valid hosts? ( The following table shows the two subnets available, the valid host range,
and the broadcast address of each: ) start at the third octet.
Subnet First Host Last Host Broadcast Address
172.16.0.0 172.16.0.1 172.16.127.254 172.16.127.255
172.16.128.0 172.16.128.1 172.16.255.254 172.16.255.255
ENGR. RUEL G. GRAFIA 3
Calculation and Computing of Subnets
Practice Example #2: 255.255.192.0 (/18)
Given: 172.16.0.0 = Network address
255.255.192.0 = Subnet mask
Required: Formula: (2n = subnet, 2n – 2 = hosts per subnet)
§ Number of subnet ?
§ Hosts per subnet ?
§ Valid Subnet or custom subnet mask ?
§ Hosts range (first host – last host) ?
§ Broadcast address ?
Solution: You need to borrow 2-bits of 1’s
§ Subnets? 22 = 4.
§ Hosts? 214 – 2 = 16,382 (6 bits in the third octet, and 8 in the fourth).
§ Valid subnets? 256 – 192 = 64. 0, 64, 128, 192. Remember that the subnetting is
performed in the third octet, so the subnet numbers are really 0.0, 64.0, 128.0, and
192.0
Alternative solution to find the valid subnet: (by applying the increment value)
The increment value is 64.
§ Broadcast address for each subnet?
172.16.63.255 , 172.16.127.255, 172.16.191.255, 172.16.255.255
ENGR. RUEL G. GRAFIA 4
Calculation and Computing of Subnets
§ Valid hosts? (The following table shows the four subnets available, the valid host range,
and the broadcast address of each): start at the third octet.
Subnet First Host Last Host Broadcast Address
172.16.0.0 172.16.0.1 172.16.63.254 172.16.63.255
172.16.64.0 172.16.64.1 172.16.127.254 172.16.127.255
172.16.128.0 172.16.128.1 172.16.191.254 172.16.191.255
172.16.192.0 172.16.192.1 172.16.255.254 172.16.255.255
Homework #2b : 255.255.255.240 (/28)
Given:
172.16.0.0 = Network address
255.255.252.0 = Subnet mask
Required:
§ Subnets?
§ Valid subnets?
§ Broadcast address for each subnet?
§ Valid hosts?
Provide a table showing the available subnets, first hosts, last host, and broadcast addresses
from a Class B 255.255.252.0 mask.
Subnet First Host Last Host Broadcast Address
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Calculation and Computing of Subnets
ENGR. RUEL G. GRAFIA 6
