Internet Protocol, Version 4

• An IPv4 address identifies a device on the Internet

  Layer 3 (network) address

• 32 bits long (4 octets): a.b.c.d

  Example: 171.64.64.64

  Example: 128.30.76.82

  Example: 12.22.58.30

• Netmask: apply this mask, if it matches, in the same network

  Netmask of 255.255.255.0 means if the first 24 bits match

  Netmask of 255.255.252.0 means if the first 22 bits match

  Netmask of 255.128.0.0 means if the first 9 bits match

  Smaller netmask (fewer 1s) means larger network

Address Structure

Historical

Originally hierarchical: network + host

• Network to get to correct network (administrative domain)
• Host to get to correct device in network (within administrative domain)

Originally 3 classes of addresses:

Today

Still assign contiguous ranges of addresses to nearby networks

<aside> 📖 Classless Inter-Domain Routing (CIDR)

Address block is a pair: address,count

Counts are powers of 2, specify netmask length

• Examples </aside>

Addresses today are managed and allocated in terms of CIDR blocks, whose size is defined by their prefix length.

A shorter prefix is a larger block than a longer prefix.

IPv4 Address Assignment