Transcript Internet Networking – Spring 2002

Reminding - ARP
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Two machines on a given network can
communicate only if they know each other’s
physical network address
ARP (Address Resolution Protocol) serves for
mapping from high-level IP address into low
level MAC address.
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Reminding – ARP
ARP Response (00:11:22:33:44:55:12)
ARP Request (10.4.10.90)
10.4.10.90
10.4.10.91
10.4.10.86
10.4.10.20
10.4.10.23
00:11:22:33:44:55:12
00:11:22:33:44:55:bc
00:11:22:33:44:55:2e
00:11:22:33:44:55:6b
00:11:22:33:44:55:40
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Proxy ARP

Proxy ARP (also called promiscuous ARP or
ARP hack) is a technique used to map a
single IP network prefix into two physical
addresses.
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Using the same network address space for more
than one physical address
Assume that there are 2 networks A and B
connected by router R that runs Proxy ARP
Using Proxy ARP, R can use the same net id
for both networks.
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Proxy ARP
Network A: 10.10.1.x/24
10.10.1.3
10.10.1.4
R
PPP
.
Network B:
10.10.1.x/24
10.10.1.5
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Proxy ARP

Router R replies to ARP requests that are
generated by hosts on the PPP connection
(Network B), in which the target IP is on
network A, namely it sends its MAC address.
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R know which hosts are connected through the
PPP.
These host assume that the destination host
are on the same physical network.
In their ARP table the router MAC address is
associated with the destination IP address.
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