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The Second Internet


recipient’s IP address = Alice’s IPv4 address (from the request)
recipient’s MAC address = Alice’s MAC address (from the request)
Only Alice gets the response (this is not a broadcast – Bob knew who made the request and sent the
response only to the requestor). Alice sees that this is a RESPONSE, and the sender’s address tells it who
the response was from. Alice then adds Bob’s IP address and MAC address into her MAC table. Now that
she knows how to send things to Bob, she goes ahead with sending the packet that she originally was
trying to send. This process is called Address Resolution. Hence the name Address Resolution Protocol.
The MAC table has expiration times (TTL), and when an entry becomes “stale” it will be discarded, and
the next time a packet is sent to that address, a new fresh entry will be added to the MAC table.
In Windows, you can view your ARP table at any time, in a DOS Window, with the command “arp –a”.
The results might look something like this:
C:\Users\lhughes.HUGHESNET>arp -a
Interface: 172.20.2.1 --- 0xb
Internet Address Physical Address Type
172.20.0.1 00-1b-21-1d-c1-59 dynamic
172.20.0.11 00-17-a4-ec-11-9c dynamic
172.20.0.12 00-e0-81-47-fa-ce dynamic
172.20.0.13 00-15-f2-2e-b4-1c dynamic
172.20.0.21 00-18-f3-2e-32-87 dynamic
172.20.0.88 00-14-fd-12-fa-5a dynamic
172.20.1.6 00-1e-90-1e-5b-4f dynamic
172.20.1.8 00-1e-65-97-de-e0 dynamic
172.20.1.9 00-15-f2-2e-b4-1c dynamic
172.20.255.255 ff-ff-ff-ff-ff-ff static
224.0.0.22 01-00-5e-00-00-16 static
224.0.0.252 01-00-5e-00-00-fc static
224.111.140.122 01-00-5e-6f-8c-7a static
226.196.145.70 01-00-5e-44-91-46 static
237.62.223.84 01-00-5e-3e-df-54 static
239.255.255.250 01-00-5e-7f-ff-fa static
Inverse ARP (InARP)
There is another protocol called Inverse ARP (InARP) that maps MAC addresses onto IP addresses. This is
needed only by a few network hardware devices (like ATM). It works almost exactly like ARP, except
different opcodes are used and the sender sends the recipient’s MAC address (which it knows) but zero
fills the recipient’s IP address (which it wants to know). The recipient recognizes its own MAC address
and responds with the same information that it does to an ARP. The older RARP (Reverse ARP) protocol
is now deprecated.
3.3.3 – Types of IPv4 Packet Transmissions
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