EtherType

The original Xerox Version 1 Ethernet network standard had a 16 bit length field, although the maximum length of a packet was 1500 bytes. This length field was soon re-used in the Version 2 Ethernet protocol as a sub-protocol label field called the EtherType, with the convention that values between 0 and 1500 indicated the use of the original Ethernet format with a length field, but higher values indicated the use of the new frame format with an EtherType sub-protocol identifier.

With the advent of the IEEE 802 suite of standards, the SNAP header is now sometimes used to transmit the EtherType of the packet for standards such as the Internet Protocol that originally used the earlier Ethernet standard. However, Ethernet Version 2 is still the most common for IP-networks.

External references:

• http://standards.ieee.org/regauth/ethertype/type-tut.html
• http://www.wildpackets.com/compendium/EN/EN-FrFmt.html