Network Masks

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A 32-bit network mask is used to distinguish the network ID and host ID in an IPv4 address.

• conceals the host ID portion of the IP address
• reveals the network ID portion
• mask and IP address are the same number of bits
• where there is a 1 in the network mask, the corresponding binary digit in the IPv4 address is part of the network ID
• 1s in the mask are always contiguous
  • E.g.,
    • valid mask: 11111111 11111111 11111111 00000000
    • invalid mask: 11111111 00000000 11111111 00000000
• network ID portion of an IP is revealed by ANDing the mask to the IPv4 address
  • when two 1s are ANDed together, the result is 1
  • any other combo = 0

Determine the Network ID

host IPv4 address = 198.51.100.1 and mask = 255.255.255.0

• first convert IP and mask to binary notation
• then AND the two binary numbers
• then convert back to dotted decimal notation

• network can be identified using prefix or slash notation
  • prefix is the number of bits set to 1
  • E.g., 198.51.100.0/24
• default masks align with octet boundaries
  • values in the dotted decimal mask will be 255 or 0
  • E.g., default 24-bit mask:
    
  • 8-bit mask = 255.0.0.0
  • 16-bit mask = 255.255.0.0
• longer network portion allows for more network IDs but fewer host IDs
  • E.g., 255.255.255.0
  • /24 network has only 254 possible host addresses
  • /8 allows for millions of hosts but only 126 network addresses