How to calculate network and broadcast address

If you are going to configure a network, then you need to know how to distribute it. To do this, you need to know the network and broadcast addresses of the network. Follow the steps below to find out how to compute these addresses if you have an IP address and subnet mask.

Steps

Method 1 of 3: For classful addressing

Step 1. For a class-based network, the total number of bits is 8

Or T_b = 8.

• The subnet mask can be 0, 128, 192, 224, 240, 248, 252, 254 and 255.

• 

• The table below allows you to determine the "number of bits used for subnets" (n) for the corresponding subnet mask.

• 

• The default subnet mask value is 255. It is not used to separate subnets.

• Example:

  Let the IP address be 210.1.1.100 and the subnet mask 255.255.255.224

  Total number of bits T_b = 8 Number of bits used for subnets n = 3 (since the subnet mask is 224 and the corresponding "number of bits used for subnets" from the table above is 3)

  

Step 2. From the previous step, you have the "number of bits used for subnets" (n) and you know T_b.

Now you can find the "number of bits remaining for hosts" (m) equal to T_b - n, since the total number of bits is the sum of the bits for subnets and hosts T_b = m + n.

• Number of bits remaining for hosts = m = T_b - n = 8 - 3 = 5

  

Step 3. Now you need to calculate the "number of subnets" equal to 2, and "the value of the last bit used for the subnet mask", which is 2^m.

The number of hosts per subnet is 2^m - 2.

• Number of subnets = 2 = 2³ = 8

  The last bit used for the subnet mask = Δ = 2^m = 2⁵ = 32

  

Step 4. Now you can find the previously calculated number of subnets by dividing them by the "last bit used for subnet mask" value or Δ-address

• The 8 subnets (as we calculated in the previous step) are shown above.
• Each of them has 32 addresses.

Step 5. Now you need to determine which network your IP address is on

The first address on this subnet will be the network address, and the last will be the broadcast address.

• Here we have selected the IP address 210.1.1.100. It is on the subnet 210.1.1.96 - 210.1.1.127 (see the previous table). Therefore, 210.1.1.96 is the network address, and 210.1.1.127 is the broadcast address for the selected IP address 210.1.1.100.

• 

Method 2 of 3: For Classless Addressing (CIDR)

Step 1. In CIDR networks, the IP address is followed by a one-bit subnet prefix, separated by a forward slash (/)

you will need to convert it to dotted quad format. Follow the steps below to do this.

1. Write down the prefix in the format shown below.

2. 

   • If the prefix is 27, write it down as 8 + 8 + 8 + 3.
   • If it's 12, write it down as 8 + 4 + 0 + 0.
   • By default, it is 32, which is written as 8 + 8 + 8 + 8.
3. Convert the corresponding bits using the table below and write the value in four-part format.

4. 

5. Let our IP address be 170.1.0.0/26. Using the table above, you can write:

										26	=	8	+	8	+	8	+	2
												255	.	255	.	255	.	192

The IP address is now 170.1.0.0, and the subnet mask is in four-part dotted format 255.255.255.192.



Step 2. Total number of bits = T_b = 8.

• The subnet mask can be 0, 128, 192, 224, 240, 248, 252, 254 and 255.
• The table below allows you to determine the "number of bits used for subnets" (n) for the corresponding subnet mask.

• 

• The default subnet mask value is 255. It is not used to separate subnets.

• From the previous step, our IP is 170.1.0.0 and our subnet mask is 255.255.255.192

  Total number of bits = T_b = 8 Number of bits used for subnets = n = 2 (since the subnet mask is 192 and the corresponding "number of bits used for subnets" from the table above is 2).

  



Step 3. From the previous step, you have the "number of bits used for subnets" (n) and you know T_b.

Now you can find the "number of bits remaining for hosts" (m) equal to T_b - n, since the total number of bits is the sum of the bits for subnets and hosts T_b = m + n.

• Number of bits remaining for hosts = m = T_b - n = 8 - 2 = 6



Step 4. Now you need to calculate the "number of subnets" equal to 2, and "the value of the last bit used for the subnet mask", which is 2^m.

The number of hosts per subnet is 2^m - 2.

• Number of subnets = 2 = 2² = 4

  The last bit used for the subnet mask = Δ = 2^m = 2⁶ = 64

  

Step 5. Now you can find the previously calculated number of subnets by dividing them by the "last bit used for subnet mask" value or Δ-address

• We get 4 subnets (as we calculated in the previous step)

• 

• Each of them has 64 addresses.

• 



Step 6. Now you need to determine which network your IP address is on

The first address on this subnet will be the network address, and the last will be the broadcast address.

• Here we have selected the IP address 170.1.0.0. It is on subnet 170.1.0.0 - 170.1.0.63 (see previous table). Therefore, 170.1.0.0 is the network address, and 170.1.0.63 is the broadcast address for the selected IP address 170.1.0.0.

• 

Method 3 of 3: Using an online calculator

Step 1. Find the IP address and subnet mask

On a Windows computer, you can do this by entering the "ipconfig" command (without quotes) at a command prompt. The IP address will appear next to the IPv4 address, and the subnet mask can be found on the line below. On a Mac, you can find the IP address and subnet mask in System Preferences under Network. Step 2. Go to

You can use any browser, regardless of which system your computer is running. Step 3. In the "IP address" field, enter the appropriate values.

The website will try to automatically detect your network address. Make sure the values are correct. Otherwise, enter the correct address. Step 4. In the "Mask" field, enter the subnet mask.

Again, the site will automatically try to calculate these values. Make sure the data is correct. The subnet mask can be entered in either CIDR format (24) or four-part dotted format (255.255.255.0). Step 5. Click Calculate.

It's a blue button opposite the Mask field. The network address will be listed below in the “Network” section, and the broadcast address will be listed in the “Broadcast” section.

Examples of

For class addressing

• IP Address = 100.5.150.34 and Subnet Mask = 255.255.240.0

  Total number of bits = T_b = 8

  Subnet mask	0	128	192	224	240	248	252	254	255
  Number of bits used for subnets (n)	0	1	2	3	4	5	6	7	8

  Number of bits used for subnetting for mask 240 = n₁ = 4

  (since the subnet mask is 240 and the corresponding "number of bits used for subnets" from the table above is 4)

  Number of bits used for subnetting for mask 0 = n₁ = 0

  (since the subnet mask is 0 and the corresponding "number of bits used for subnets" from the table above is 0)

  Number of bits remaining for hosts for mask 240 = m₁ = T_b - n₁ = 8 - 4 = 4

  Number of bits remaining for hosts for mask 0 = m₂ = T_b - n₂ = 8 - 0 = 8

  Number of subnets for a mask 240 = 2 ^₁ = 2⁴ = 16

  Number of subnets for mask 0 = 2 ^₂ = 2⁰ = 1

  The last bit used for the subnet mask for mask 240 = Δ₁ = 2^m₁ = 2⁴ = 16

  The value of the last bit used for the subnet mask for mask 0 = Δ₂ = 2^m₂ = 2⁸ = 256

  For subnet mask 240, the addresses will be split by 16, and for mask 0, there will be 256. Using the values of Δ₁ and Δ₂, we get 16 subnets below

  100.5.0.0 - 100.5.15.255	100.5.16.0 - 100.5.31.255	100.5.32.0 - 100.5.47.255	100.5.48.0 - 100.5.63.255
  100.5.64.0 - 100.5.79.255	100.5.80.0 - 100.5.95.255	100.5.96.0 - 100.5.111.255	100.5.112.0 - 100.5.127.255
  100.5.128.0 - 100.5.143.255	100.5.144.0 - 100.5.159.255	100.5.160.0 - 100.5.175.255	100.5.176.0 - 100.5.191.255
  100.5.192.0 - 100.5.207.255	100.5.208.0 - 100.5.223.255	100.5.224.0 - 100.5.239.255	100.5.240.0 - 100.5.255.255

  The IP address 100.5.150.34 belongs to the subnet 100.5.144.0 - 100.5.159.255, so 100.5.144.0 is the network address and 100.5.159.255 is the broadcast address

For Classless Addressing (CIDR)

• IP address in the CIDR network = 200.222.5.100/9

										9	=	8	+	1	+	0	+	0
												255	.	128	.	0	.	0

IP Address = 200.222.5.100 and Subnet Mask = 255.128.0.0

Total number of bits = T_b = 8

Subnet mask	0	128	192	224	240	248	252	254	255
Number of bits used for subnets (n)	0	1	2	3	4	5	6	7	8

Number of bits used for subnetting for mask 128 = n₁ = 1

(since the subnet mask is 128 and the corresponding "number of bits used for subnets" from the table above is 1)

Number of bits used for subnetting for mask 0 = n₂ = n₃ = 0

(since the subnet mask is 0 and the corresponding "number of bits used for subnets" from the table above is 0)

Number of bits remaining for hosts for mask 128 = m₁ = T_b - n₁ = 8 - 1 = 7

Number of bits remaining for hosts for mask 0 = m₂ = m₃ = T_b - n₂ = T_b - n₃ = 8 - 0 = 8

Number of subnets for mask 128 = 2 ^₁ = 2¹ = 2

Number of subnets for mask 0 = 2 ^₂ = 2 ^₃ = 2⁰ = 1

The last bit used for the subnet mask for mask 128 = Δ₁ = 2^m₁ = 2⁷ = 128

Number of hosts per subnet = 2^m₁ - 2 = 2⁷ - 2 = 126

The value of the last bit used for the subnet mask for mask 0 = Δ₂ = Δ₃ = 2^m₂ = 2^m₃ = 2⁸ = 256

Number of hosts per subnet with mask 0 = 2^m₂ - 2 = 2^m₃ - 2 = 2⁸ - 2 = 254

For subnet mask 128, the addresses will be split by 128, and for mask 0 there will be 256. Using the values of Δ₁ and Δ₂, we get 2 subnets below

200.0.0.0 - 200.127.255.255

200.128.0.0 - 200.255.255.255

The IP address 200.222.5.100 belongs to the subnet 200.128.0.0 - 200.255.255.255, and therefore 200.128.0.0 is the subnet address and 200.255.255.255 is the broadcast address

Advice

• On CIDR networks, once you have converted the prefix to four-part format, you can use the same method as for class-based networks.
• This method only works for IPv4 type networks and is not suitable for IPv6.