Identifying the Network and Broadcast Address of a Subnet
In this lesson we will attempt to simplify the identification of the Network and Broadcast address using a known IP address, within the network or subnet, and the CIDR or Netmask. In this lesson we will walk you through the terms you need to know, the basic math and some examples.
Terms you need to know:
CIDR: Classless Inter-Domain Routing. Think of it as a replacement for a Netmask. The CIDR Value is equivalent to the number of on bits in a 32 bit address going left to right. For example: the CIDR value of 24 means the first 24 bits are turned on and the last 8 bits are turned off: 11111111.11111111.11111111.00000000. (See RFC’s: 1519, 1817, 4632).
Network Address (or Network ID): This is the address that identifies the subnet of a host.
Broadcast Address: An IP Address that allows information to be sent to all machines on a given subnet rather than a specific machine. (See RFCs: 826, 919, 922, 947, 1027, 1770, 3021).
Binary: A base 2 numbering system (machine language).
Bitwise AND Operator: Represented by the “&” symbol, the Bitwise AND Operator returns a one in each bit position if both corresponding bits are one. Example: x & y = z.
Binary Inversion: In a Binary CIDR or Netmask we are inverting the ones to zeros and the zeros to ones.
Bitwise OR Operator: Represented by the “|” symbol, the Bitwise OR Operator returns a 1 in each bit position if one or both corresponding bits are one.
The Steps to identify the Network and Broadcast Address of a Subnet
Convert the IP Address and CIDR (or Netmask) to binary. In our lesson entitled Decimal and Binary Conversion of IP Addresses we gave you the tools to convert any IP to Binary. If you need additional help you can try our handy IP Conversion Calculators.
Use a Bitwise AND (IP & CIDR) Operator to return the corresponding values of the IP and CIDR addresses. This gives you the Network Address (Network ID) A simple way to use the Bitwise AND Operator in Binary is show in the following example:
IP Address: 192.168.1.15
CIDR: 24 (Netmask: 255.255.255.0)
Binary IP Address: 11000000.10101000.00000001.00001111
Binary CIDR: 11111111.11111111.11111111.00000000
Using the Bitwise AND (&) Operator, compare the Binary IP Address to the Binary CIDR Address. The result will be the Network Address of the IP Address we are using:
Binary IP: 11000000.10101000.00000001.00001111
Binary CIDR: 11111111.11111111.11111111.00000000
Binary Network: 11000000.10101000.00000001.00000000
The resultant Network Address is 11000000.10101000.00000001.00000000. Converting this back to the format of an IPv4 Address gives us 192.168.1.0. This is our Network Address. Therefore, 192.168.1.15 belongs to the 192.168.1.0/24 network.
To get the Broadcast Address we need to do a Binary inversion of the CIDR or Netmask Address.
The inversion of the CIDR Address of 11111111.11111111.11111111.00000000 becomes: 00000000.00000000.00000000.11111111.
Now we use the Bitwise OR Operator on the Binary Network Address and the inverted CIDR Address to get the Broadcast address.
Binary Network Address: 11000000.10101000.00000001.00000000 Inverted Binary CIDR: 00000000.00000000.00000000.11111111 Binary Broadcast Address: 11000000.10101000.00000001.11111111
We now convert 11000000.10101000.00000001.11111111 to IPv4 Decimal octet: 192.168.1.255.
The Broadcast Address for the 192.168.1.0/24 Subnet is 192.168.1.255.
Now that you have your feet wet, let’s try a few more.
Identify the Network and Broadcast Addresses for each of the following examples:
- 10.10.1.97/23
- 192.168.0.3/25
- 172.16.5.34/26
- 192.168.11.17/28
Example one: Convert 10.10.1.97/23 to Binary.
IP Address: 00001010.00001010.00000001.01100001
CIDR Address: 11111111.11111111.11111110.00000000
Use Bitwise AND Operator (IP & CIDR):
IP Address: 00001010.00001010.00000001.01100001 CIDR Address: 11111111.11111111.11111110.00000000 Network Address: 00001010.00001010.00000000.00000000
Network Address: 10.10.0.0
Binary Inversion of CIDR:
Binary CIDR: 11111111.11111111.11111110.00000000 Inverted Binary CIDR: 00000000.00000000.00000001.11111111
Use Bitwise OR Operator to get the Broadcast Address:
Binary Network: 00001010.00001010.00000000.00000000
Inverted Binary CIDR: 00000000.00000000.00000001.11111111
Binary Broadcast: 00001010.00001010.00000001.11111111
Broadcast Address: 10.10.1.255
IP Address 10.10.1.97/23 belongs to the 10.10.0.0/23 Network. The network Address is 10.10.0.0 and the Broadcast Address is 10.10.1.255.
Example two: Convert 192.168.0.3/25 to Binary.
IP Address: 11000000.10101000.00000000.00000011
CIDR Address: 11111111.11111111.11111111.10000000
Use Bitwise AND Operator (IP & CIDR):
IP: 11000000.10101000.00000000.00000011 CIDR: 11111111.11111111.11111111.10000000 Network: 11000000.10101000.00000000.00000000
Network Address: 192.168.0.0
Binary Inversion of CIDR:
Binary CIDR: 11111111.11111111.11111111.10000000 Inverted Binary CIDR: 00000000.00000000.00000000.01111111
Use Bitwise OR Operator to get the Broadcast Address:
Binary Network: 11000000.10101000.00000000.00000000
Inverted Binary CIDR: 00000000.00000000.00000000.01111111
Binary Broadcast: 11000000.10101000.00000000.01111111
Broadcast Address: 192.168.0.127
IP Address 192.168.0.3/25 belongs to the 192.168.0.0/25 Network. The network Address is 192.168.0.0 and the Broadcast Address is 192.168.0.127.
Example three: Convert 172.16.5.34/26 to Binary.
IP Address: 11000000.10101000.00000000.00000011
CIDR Address: 11111111.11111111.11111111.10000000
Use Bitwise AND Operator (IP & CIDR):
IP: 10101100.00010000.00000101.00100010 CIDR: 11111111.11111111.11111111.11000000 Network: 10101100.00010000.00000101.00000000
Network Address: 172.16.5.0
Binary Inversion of CIDR:
Binary CIDR: 11111111.11111111.11111111.11000000 Inverted Binary CIDR: 00000000.00000000.00000000.00111111
Use Bitwise OR Operator to get the Broadcast Address:
Binary Network: 10101100.00010000.00000101.00000000
Inverted Binary CIDR: 00000000.00000000.00000000.00111111
Binary Broadcast: 10101100.00010000.00000101.00111111
Broadcast Address: 172.16.5.63
IP Address 172.16.5.34/26 belongs to the 172.16.5.0/26 Network. The network Address is 172.16.5.0 and the Broadcast Address is 172.16.5.63.
Example four: Convert 192.168.11.17/28 to Binary.
IP Address: 11000000.10101000.00001011.00010001
CIDR Address: 11111111.11111111.11111111.11110000
Use Bitwise AND Operator (IP & CIDR):
IP: 11000000.10101000.00001011.00010001 CIDR: 11111111.11111111.11111111.11110000 Network: 11000000.10101000.00001011.00010000
Network Address: 192.168.11.16
Binary Inversion of CIDR:
Binary CIDR: 11111111.11111111.11111111.11110000 Inverted Binary CIDR: 00000000.00000000.00000000.00001111
Use Bitwise OR Operator to get the Broadcast Address:
Binary Network: 11000000.10101000.00001011.00010000
Inverted Binary CIDR: 00000000.00000000.00000000.00001111
Binary Broadcast: 11000000.10101000.00001011.00011111
Broadcast Address: 192.168.11.31
IP Address 192.168.11.17/28 belongs to the 192.168.11.16/28 Network. The network Address is 192.168.11.16 and the Broadcast Address is 192.168.11.31.
Coming soon: Learning to Subnet the Easy Way
