Information Technology Reference
In-Depth Information
Features and Functions of Ethernet
The Data Link Layer of the TCP/IP Model controls the transfer of data across
a network media. In an Ethernet Local Area Network (LAN), the Ethernet
frame is used at the Data Link Layer to package and transmit the data.
The hosts on an Ethernet LAN are assigned unique addresses that are asso-
ciated with the Data Link Layer. These addresses are referred to as
Ethernet
addresses,
hardware
addresses,
physical
addresses, and/or
Media Access Control
(MAC)
addresses.
Typically, these addresses are assigned by the company that manufactures the
Network Interface Card (NIC), and a unique address is associated with each
NIC. Thus, the Ethernet address is associated with the NIC, not with a sys-
tem. A system with two NICs (such as a router that is attached to two net-
works) will have two Ethernet addresses. The addresses are managed by the
Institute of Electrical and Electronics Engineers (IEEE) and are distributed
to manufacturers on an as-needed basis.
The Ethernet address consists of 48 bits (six bytes of data). Typically an
Ethernet address is shown as six 2-digit hexadecimal numbers separated by
colons. For example,
00:40:05:37:9d:db
is the Ethernet address for the sys-
tem used to generate most of the listings in this topic.
The Ethernet frame transmitted on the LAN contains the Ethernet address
of the NIC that should receive the frame. All hosts on a LAN receive all
transmitted Ethernet frames. Any received frames with an Ethernet address
that matches the Ethernet address of the NIC are read and processed.
The Ethernet addresses are only used on the LAN and do not pass through
routers. Thus, any Ethernet frame being sent to a host that is not on the cur-
rent LAN will use the Ethernet address of the destination host's closest
router as its destination. Likewise, an Ethernet frame being sent from a
router to a host will have the Ethernet address of the router as its source.
However, most TCP/IP applications address hosts using IP addresses.
Typically, the IP address needs to be converted to an Ethernet address.
Because each host has a unique IP address and a unique Ethernet address, a
process is needed to map between these two addresses.
The Address Resolution Protocol (ARP) and the Reverse
ARP (RARP)
This process to map between Ethernet and IP addresses is referred to as
address
resolution
. The protocol used to map from an IP address to an Ethernet address
is referred to as the Address Resolution Protocol (ARP). The details of this
process are specified in
RFC 826—An Ethernet Address Resolution Protocol
.
