User Guide

26 NetWare TCP/IP Administration Guide
NetWare TCP/IP Administration Guide
103-000151-001
August 30, 2001
Novell Confidential
Manual 99a38 July 17, 2001
Physical and IP Addresses
Each node has a physical address for the specific hardware device that
connects it to a network. For instance, a physical address on an Ethernet
network is a 6-byte numeric value, such as 08-00-14-57-69-69. It is assigned
by the manufacturer of the Ethernet interface hardware. X.25 networks, which
conform to the specification of the ITU-T (International Telecommunications
Union, Telecommunications sector), previously CCITT, use the X.121
standard for physical addresses, which consist of 14-digit numbers.
NOTE: Physical addresses are also called media access control (MAC)
addresses. Throughout the rest of this section, all references to MAC or physical
addresses assume physical addresses on Ethernet, token ring, or FDDI networks.
Since IP uses a 32-bit address and Ethernet uses a 48-bit Ethernet address
there is a conflict. To associate the IP address to a physical address on an
Ethernet network, a mapping must occur between the two types. The address
resolution protocol (ARP) provides a mapping between the two different
forms of addresses.
IP Address to Physical Address Translation
Each physical medium has its own physical address for nodes on that medium.
The physical addresses are also called MAC addresses. Ethernet and token
ring networks use 6-byte MAC addresses. ARCnet uses a 1-byte MAC
address.
IP addresses are independent of the hardware. When an IP packet is
transmitted on the network, it is first encapsulated within the physical frame
used by that network. Figure 9 shows an IP packet encapsulated in an Ethernet
frame. The IP packet contains an Internet address for a node, but the Ethernet
frame must have a physical address for it to be delivered on the data-link
network. Therefore, the sending node must be able to map an IP address to a
physical hardware address.
Figure 9 A packet encapsulated in an Ethernet frame
Preamble
Destination
Address
Source
Address
Packet
Type
Packet Data
Ethernet
CRC
IP Datagram
Frame Data
Frame Header
Ethernet Frame