Qtech QSW-3900-48-SFP-DC [163/245] Brief introduction of arp
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Qtech QSW-3900-48-SFP-DC [163/245] Brief introduction of arp

Qtech QSW-3900-48-SFP-DC [163/245] Brief introduction of arp
QTECH Software Configuration Manual
12-162
Chapter 12 ARP Configuration
12.1 Brief Introduction of ARP
In computer networking, the Address Resolution Protocol (ARP) is the method for finding a host's link
layer (hardware) address when only its Internet Layer (IP) or some other Network Layer address is known. ARP is
defined in RFC 826 It is Internet Standard STD 37.
ARP has been implemented in many types of networks; it is not an IP-only or Ethernet-only protocol. It can
be used to resolve many different network layer protocol addresses to interface hardware addresses, although, due to
the overwhelming prevalence of IPv4 and Ethernet, ARP is primarily used to translate IP addresses to Ethernet MAC
addresses. It is also used for IP over other LAN technologies, such as Token Ring, FDDI, or IEEE 802.11, and for IP
over ATM.
In the next generation Internet Protocol, IPv6, ARP's functionality is provided by the Neighbor Discovery
Protocol (NDP).
ARP is a Link Layer protocol because it only operates on the local area network or point-to-point link that a
host is connected to.
ARP is also very often discussed in terms of the Open Systems Interconnect (OSI) networking model,
because that model addresses hardware-to-software interfaces more explicitly and is preferred by some equipment
manufacturers. However, ARP was not developed based on the design principles and strict encapsulation hierarchy of
this model and, therefore, such discussions create a number of conflicts as to the exact operating layer within this
model. Most often ARP is placed into the Data Link Layer (Layer 2), but it also requires the definitions of network
The following is the packet structure used for ARP requests and replies. On Ethernet networks, these
packets use an EtherType of 0x0806, and are sent to the broadcast MAC address of FF : FF : FF : FF : FF : FF. Note
that the EtherType (0x0806) is used in the Ethernet header, and should not be used as the PTYPE of the ARP packet.
The ARP type (0x0806) should never be used in the PTYPE field of an ARP packet, since a hardware protocol
address should never be linked to the ARP protocol. Note that the packet structure shown in the table has SHA and
THA as 48-bit fields and SPA and TPA as 32-bit fields but this is just for convenience — their actual lengths are
determined by the hardware & protocol length fields.
bit
offset
0 - 7 8 - 15 16 - 31
0
Hardware type (HTYPE) Protocol type (PTYPE)
32
Hardware length
(HLEN)
Protocol length (PLEN) Operation (OPER)
64
Sender hardware address (SHA) (first 32 bits)
96
Sender hardware address (SHA) (last 16 bits)
Sender protocol address (SPA) (first 16 bits)
128
Sender protocol address (SPA) (last 16 bits) Target hardware address (THA) (first 16 bits)
160
Target hardware address (THA) (last 32 bits)
192
Target protocol address (TPA)
Ø Hardware type (HTYPE)
Each data link layer protocol is assigned a number used in this field. For example, Ethernet is 1.
Ø Protocol type (PTYPE)
Each protocol is assigned a number used in this field. For example, IP is 0x0800.
Ø Hardware length (HLEN)
Length in bytes of a hardware address. Ethernet addresses are 6 bytes long.
Ø Protocol length (PLEN)
Length in bytes of a logical address. IPv4 address are 4 bytes long.
Ø Operation
Specifies the operation the sender is performing : 1 for request, 2 for reply, 3 for RARP request, and 4 for
RARP reply.
Ø Sender hardware address (SHA)
Hardware address of the sender.

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