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Monday, April 21, 2008

Understanding 32-bit IP

IP addresses are just 32-bit numbers. Subnet masks are just a "cover" that can be arbitrarily slid up and down the IP address's bits to create larger or smaller networks.

Focusing on 32-bit IPv4 addresses themselves now, there are a few different types that need to be understood. All IP addresses can be in the range to, but some have special uses.

Packets that will not leave the host (i.e. they will not traverse an external network interface). Example:

Packets that are destined for a single IP address. Example:

Packets that will be duplicated by the router, and eventually routed by multicast routing mechanisms. Example:

Limited Broadcast:
A broadcast packet, sent to every host, limited to the local subnet. Example:

Directed Broadcast:
Packets that are routed to a specific subnet, and then broadcast. Example, assuming we are not on this subnet:

There are also some special cases of IP addresses, including private and multicast addresses. Addresses in the range - are reserved for multicast. Everything below that range is fair game on the Internet, excluding addresses reserved by RFC 1918 and a few other special-purpose assignments. These "1918 addresses" are private addresses, meaning Internet routers will not route them. The ranges include:

  • -
  • -
  • -

    These IP addresses can be assigned locally to as many computers as you want, but before those computers access the Internet, the addresses must be translated to a globally routable address. This is commonly done via Network Address Translation ( NAT ). The 1918 addresses aren't the only reserved spaces, but they are defined to be "site local." Multicast also has a reserved range of addresses that aren't designed to escape onto the Internet: - are multicast "link-local" addresses.

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