Ethernet

IEEE 802.3 — the data link layer frame format

Frame Structure IEEE 802.3

7B preamble 1B sfd 6 bytes dest mac 6 bytes src mac 2B ethertype 46-1500 bytes payload 4 bytes fcs
FieldSizeDescription
Preamble7 bytesAlternating 1/0 pattern (10101010...)
SFD1 byteStart Frame Delimiter (10101011)
Dest MAC6 bytesDestination hardware address
Src MAC6 bytesSource hardware address
EtherType2 bytesProtocol identifier
Payload46-1500 bytesUpper layer data
FCS4 bytesFrame Check Sequence (CRC-32)

EtherType Values IANA assigned

EtherTypeProtocolDescription
0x0800IPv4Internet Protocol version 4
0x0806ARPAddress Resolution Protocol
0x86DDIPv6Internet Protocol version 6
0x8100802.1QVLAN-tagged frame
0x88CCLLDPLink Layer Discovery Protocol

MAC Address Format EUI-48

AA oui : BB oui : CC oui : DD nic : EE nic : FF nic 
AA:BB:CC:DD:EE:FF
└──┬──┘ └──┬──┘
  OUI     NIC
(vendor) (device)
ComponentSizeDescription
OUI3 bytesOrganizationally Unique Identifier (vendor)
NIC3 bytesNetwork Interface Controller (device specific)

Special MAC Addresses reserved

AddressTypeDescription
FF:FF:FF:FF:FF:FFBroadcastAll hosts on local network
01:00:5E:xx:xx:xxIPv4 MulticastMapped from multicast IP
33:33:xx:xx:xx:xxIPv6 MulticastMapped from multicast IPv6
00:00:00:00:00:00NullInvalid / unassigned address
bit 0 of first byte: 0 = Unicast 1 = Multicast

Frame Size Limits IEEE 802.3

64 min bytes 1500 mtu (payload) 1518 max bytes
LimitValueDescription
Minimum Frame64 bytesEnsures collision detection works
Maximum Frame1518 bytesStandard max (excl. preamble, SFD)
MTU1500 bytesMaximum Transmission Unit (payload)
Jumbo Frame9000 bytesExtended MTU (non-standard)

MTU Defaults common values

Network TypeMTUNotes
Ethernet1500 bytesStandard default
PPPoE1492 bytes8-byte PPPoE header overhead
VPN/Tunnel1400-1460 bytesVaries by encapsulation
Loopback65535 bytesLocal interface