Mask Operations

1
Mask Operations

• Masks were introduced in Chapter 3
• IP addresses alone do not tell you the size of
  their network or subnet parts
• Network Mask
• Has 1s in the network part
• Has 0s in the remaining bits
• Subnet Mask
• Has 1s in the network plus subnet parts
• Has 0s in the remaining bits

2
Mask Operations

• Based on Logical AND
• Both must be true for the result to be true
• Example
• 1010101010 Data
• 1111100000 Mask
• 1010100000 Result

3
Mask Operations

• Based on Logical AND
• If mask bit is 1, get back original data
• If mask bit is 0, bet back zero
• Example
• 1010101010 Data
• 1111100000 Mask
• 1010100000 Result

4
Mask Operations

• IP packet arrives at a router
• Router sees destination IP address
• 11111111 01000000 10101010 00000000
• Compares to each router forwarding table row
• Address Part in First Entry
• 11111111 01000000 00000000 00000000
• Mask in First Entry
• 11111111 11100000 00000000 00000000

5
Mask Operations

• Mask the IP destination Address
• 11111111 01000000 10101010 00000000 (IP address)
• 11111111 11100000 00000000 00000000 (mask)
• 11111111 01000000 00000000 00000000 (result)
• Compare Result with First Entry Address part
• 11111111 01000000 00000000 00000000 (address
  part)
• 11111111 01000000 00000000 00000000 (result)
• The Entry is a Match!

6
Mask Operations

• Recap
• Read destination IP address of incoming IP packet
• For each entry in the router forwarding table
• Read the mask (prefix)
• Mask the incoming IP address
• Compare the result with the entrys IP address
  part
• Do they match or not?

7
Mask Operations

• Simple for Computers
• Computers have circuitry to AND to numbers
• Computers have circuitry to COMPARE two numbers
  to see if they are equal or not
• Very computer-friendly, so used on routers
• Difficult for people, unfortunately