O–P

octet.

A group of eight binary digits; an octet can represent the numbers 0 to 255 in decimal.

OSI model.

The seven-layer model for designing network protocols.

partial mesh.

A network where each router has only one connection to a subset of all the other routers in the network.

passive.

The state of a route in EIGRP when the router has a successor through which to forward packets.

passive interface.

An interface on which the protocol is not running, although the link itself is advertised as reachable by the routing protocol.

PAT. Port Address Translation.

Translating source and destination address at the port level, which allows multiplexing many sessions from different hosts onto a single address. Commonly used to permit privately addressed hosts to access servers on the Internet using registered addresses.

peer group.

A group of BGP neighbors that are treated the same; a BGP router only builds one update per peer group if they are configured, rather than one update per neighbor.

permanent virtual circuit

See [PVC. permanent virtual circuit. A permanent virtual (or multiplexed) point-to-point link; common in Frame Relay, X.25, and ATM networks.]

physical layer

The physical plant, cables, and modulation methods used to transmit data in a network.

policy routing

Routing packets based on some criteria other than the destination address; choosing different paths for QoS purposes isn't generally considered policy routing.

pop.

The act of removing a label from the top of the MPLS label stack.

Port Address Translation.

See [PAT. Port Address Translation. Translating source and destination address at the port level, which allows multiplexing many sessions from different hosts onto a single address. Commonly used to permit privately addressed hosts to access servers on the Internet using registered addresses.]

prefix length.

The number of bits in the subnet mask; for instance, the subnet mask 255.255.255.0 has 24 bits set to 1 and is, therefore, a 24-bit subnet mask. The prefix length is often expressed with "/x" after the IP address.

presentation layer.

The layer in the OSI network model that is responsible for presenting data in an appropriate format to the devices that are communicating.

private address.

Address or range of addresses defined by the IETF as unusable (unroutable) on the Internet.

pseudonode.

A mechanism used in IS-IS to reduce the full mesh adjacency normally required on broadcast networks.

push.

The act of putting a new label on the top of an MPLS label stack.

PVC. permanent virtual circuit.

A permanent virtual (or multiplexed) point-to-point link; common in Frame Relay, X.25, and ATM networks.