Subnetwork Access Protocol (SNAP)

SubNetwork Access Protocol (SNAP) refers to a standard used for transmitting IP datagrams across IEEE 802 networks. This means that the IP datagrams can be routed on IEEE 802 networks encapsulated inside the SNAP data link layers 802.3, 802.4 or 802.5, physical network layers, and the 802.2 LLC. SNAP is also used with non-IEEE 802 physical network layers that make use of 802.2 LLC. SNAP is enclosed in a Logic Link Control (LLC IEEE 802.2) header extension. It is intended for the encapsulation of ARP requests and replies, and IP datagrams on IEEE 802 networks.

A SNAP header follows the LLC header and also includes an organization code, which indicates that the following 16 bits designate the Ether Type code. Typically, every communication is carried out by means of 802.2 type 1 communication. Compatible systems on similar IEEE 802 networks can make use of 802.2 type 2 communication after validating that it is assisted by both nodes. This can be done by means of the 802.2 XID mechanism. Even so, type 1 communication is the preferred method, and must be backed up by the entire implementation.

The mapping of 32-bit Internet addresses to 48-bit or 16-bit IEEE 802 addresses is accomplished through the dynamic discovery method of Address Resolution Protocol (ARP). IEEE 802 networks can have 48-bit or 16-bit physical addresses. The SNAP enables the effective use of either address size inside a specific IEEE 802 network.

The LLC header is three octets long, while the SNAP header is five octets long. So, a combination of LLC and SNAP headers is overall eight octets long. On Ethernet, this minimizes the available payload size for protocols.