Technical data

182 Agilent Connectivity Guide
7TCP/IP Network Basics
Duplicate IP Address detection is described by several Internet
standards (RFC 2642/RFC 2131/ZEROCONF). It is implemented using
ARP (Address Resolution Protocol) and uses broadcast Ethernet
communication. The scope of duplicate IP address detection is the
current Ethernet subnet. Most operating systems (Windows, Apple, etc.)
implement duplicate IP address detection.
Duplicate IP Addresses on non-DHCP Networks Generally,
duplicate IP addresses only occur on a manually configured IP address
network that does not use DHCP and AutoIP/ZEROCONF. For
example, a user may try to determine an IP address for a host by
PINGing IP addresses on the network until an IP address is selected that
does not respond.
Based on this non-response, the user may assume the IP address is
unused and assign it to their device. However, at a later date, a host that
legitimately has that IP address will attempt to use it. As a result, both
hosts will experience network problems that can be very difficult to
track down.
A duplicate IP address can also happen when a user makes an error
during manual entry of an IP address and accidentally configures an IP
address already in use elsewhere.
Duplicate IP Addresses on DHCP Networks Duplicate IP
addresses on DHCP systems are unlikely but they are possible. The
DHCP specification (RFC 2131) specifies how a duplicate IP address
check should be done within the DHCP
Discover/Offer/Request/Acknowledgement protocol sequence. Ideally,
the network interface should be disabled when a duplicate IP address is
detected. Windows informs the user of the duplicate IP address, but
does NOT disable the network interface.
If two or more devices on the network have the same IP address, the
first device that starts using the duplicate IP address will not detect a
problem because it is the only device using that IP address. However,
when a second device starts and attempts to use the duplicate IP address,
a duplicate IP address error is reported.