Because broadcasting depends on the specific data link layer in use on a local network, we must discuss it with reference to both physical networks and logical networks.
The terms we will use in referring to physical networks are, from the point of view of the host sending or forwarding a broadcast:
Local Hardware Network The physical link to which the host is attached. Remote Hardware Network A physical network which is separated from the host by at least one gateway. Collection of Hardware Networks A set of hardware networks (transitively) connected by gateways.
The IP world includes several kinds of logical network. To avoid ambiguity, we will use the following terms:
Internet The DARPA Internet collection of IP networks. IP Network One or a collection of several hardware networks that have one specific IP network number. Subnet A single member of the collection of hardware networks that compose an IP network. Host addresses on a given subnet share an IP network number with hosts on all other subnets of that IP network, but the local-address part is divided into subnet-number and host-number fields to indicate which subnet a host is on. We do not assume a particular division of the local-address part; this could vary from network to network.
The introduction of a subnet level in the addressing hierarchy is at variance with the IP specification , but as the use of addressable subnets proliferates it is obvious that a broadcasting scheme should support subnetting. For more on subnets, see .
In this paper, the term "host address" refers to the host-on-subnet address field of a subnetted IP network, or the host-part field otherwise.
An IP network may consist of a single hardware network or a collection of subnets; from the point of view of a host on another IP network, it should not matter.