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 0.0.0.0 to 255.255.255.255, but some have special uses.

Loopback:
Packets that will not leave the host (i.e. they will not traverse an external network interface). Example: 127.0.0.1

Unicast:
Packets that are destined for a single IP address. Example: 2.2.2.2

Multicast:
Packets that will be duplicated by the router, and eventually routed by multicast routing mechanisms. Example: 226.0.0.2

Limited Broadcast:
A broadcast packet, sent to every host, limited to the local subnet. Example: 255.255.255.255

Directed Broadcast:
Packets that are routed to a specific subnet, and then broadcast. Example, assuming we are not on this subnet: 1.1.1.255

There are also some special cases of IP addresses, including private and multicast addresses. Addresses in the range 224.0.0.0 - 239.255.255.255 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:

10.0.0.0 -10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255 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: 224.0.0.0 - 224.0.0.255 are multicast "link-local" addresses.