Masks

Some software reports network masks using dotted decimal notation (e.g. 255.255.255.0) in which each 1 bit means that that bit is part of the subnet id and the 0 bits are the host id. Other software (most likely routers) uses the CIDR notation /24 to mean 24 bits are subnet id.

Convert these network masks to CIDR notation: The first one is done for you.

Network Mask	CIDR Notation
255.255.255.0	/24
255.255.252.0	/_____
255.255.0.0	/_____
255.0.0.0	/_____
255.255.240.0	/_____

Converting decimal to binary

There are two ways to convert decimal to binary, manual and using a calculator.

Manual

Divide by 16. This gives two base 16 numbers, the quotient and remainder. Convert each of these to binary. The quotient is the left most 4 bits and remainder the rightmost:

Use a calculator

Calculators include:

1. The LINUX "bc" command.
   • Type bc and press ENTER.
   • Type obase=2 and press ENTER (instructs bc to output numbers in base 2 - binary)
   • Type your decimal number and press ENTER. bc will display it in binary.
2. A phone app like the andoroid "RealCalc" calculator.
3. A scientific calculator.

CIDR subnets

Create subnet masks in dotted decimal format for these subnets in CIDR notation. The first is done for you.

CIDR subnet ID	Netmask
60.40.10.0/24	255.255.255.0
60.40.10.0/16	______.______.______.______
60.40.10.220/28	______.______.______.______
10.20.30.252/30	______.______.______.______

CIDR IP addresses

For each of the following CIDR format IP addresses, figure out the subnet ID and the host ID within that subnet. The first one is done for you.

CIDR IP Address	Subnet ID	Host ID
130.150.10.50/24	130.150.10.0	50
6.25.98.4/8	_____._____._____._____	_____________
6.25.98.4/16	_____._____._____._____	_____________
6.25.98.151/28	_____._____._____._____	_____________

An Example

Go to a Windows machine and using the command

ipconfig /all | more

• IP address ___ ___ ___ ___

• Network Mask ___ ___ ___ ___

• DHCP Server ___ ___ ___ ___

• Default gateway (router) ___ ___ ___ ___

• DNS Server ___ ___ ___ ___

Answer these questions:

1. How many bits are in the subnet address? ______

2. Write the subnet address in CIDR format. ______________________

3. Is the DHCP server on the same subnet? ______________

4. Is the Default gateway on the same subnet? ________________

5. Is the DNS server on the same subnet? __________________