Type / Length Field

• For Ethernet, type of the encapsulated packet

• • IP (0x0800)
  • ARP (0x0806)
  • IPX (0x8137)

• For 802.3, length of the data field in bytes

• Most modern Ethernet NICs use the older “Ethernet” frame style

• • Length is determined by the physical layer

• Ethernet and 802.3 style packets can co-exist on the same network

• • If the type/length field is <= 1500 bytes, it’s a length, otherwise it’s a type