show the ARP cache

every network interface card (NIC)/LAN network card has a unique-as-possible vendor-asigned MAC address in the format 00:15:5d:00:07:08

this address is changeable like this:

ip link set dev eth0 down;
ip link set dev eth0 address 00:15:5d:00:07:09;
ip link set dev eth0 up;

(thanks)

WARNING! Changing your NIC’s MAC will massively reset your connectivity 😀 (switch, router, dhcp will have to relearn about those changes))

the first thing a NIC does when hooked up to a switch – it announces it’s presence – to the switch and all other attached devices by broadcasting to FF:FF:FF:FF:FF:FF (ARP broadcast = send to all address)

„hello i am 00:22:4d:6a:e5:c6 and looking for a DHCP server to get an dynamic IP asigned“

when properly configured the DHCP server answers like this

„hello 00:22:4d:6a:e5:c6 please take that IP 172.20.0.7“

so everyone on the network knows that 00:22:4d:6a:e5:c6 can be reached via TCP via IP 172.20.0.7

[root@CentOS7 user]# arp -an
? (172.20.0.2) auf 2c:76:8a:aa:60:3a [ether] auf eth0
? (172.20.0.1) auf 34:31:c4:53:37:8a [ether] auf eth0
? (172.20.0.7) auf 00:22:4d:6a:e5:c6 [ether] auf eth0
? (192.168.1.1) auf 00:15:5d:00:07:08 [ether] auf eth0
? (192.168.1.3) auf 00:15:5d:00:07:09 [ether] auf eth0

[root@CentOS7 user]#  ip neighbour show
172.20.0.2 dev eth0 lladdr 2c:76:8a:aa:60:3a STALE
172.20.0.1 dev eth0 lladdr 34:31:c4:53:37:8a STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 REACHABLE
192.168.1.1 dev eth0 lladdr 00:15:5d:00:07:08 STALE
192.168.1.3 dev eth0 lladdr 00:15:5d:00:07:09 STALE

listen to the ARP IP<->MAC messages

suse12:~ # ip monitor
172.20.0.2 dev eth0 lladdr 2c:76:8a:aa:60:3a STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 PROBE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 REACHABLE
172.20.0.2 dev eth0 lladdr 2c:76:8a:aa:60:3a STALE
172.20.0.2 dev eth0 lladdr 2c:76:8a:aa:60:3a PROBE
172.20.0.2 dev eth0 lladdr 2c:76:8a:aa:60:3a REACHABLE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 STALE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 PROBE
172.20.0.7 dev eth0 lladdr 00:22:4d:6a:e5:c6 REACHABLE
172.20.0.2 dev eth0 lladdr 2c:76:8a:aa:60:3a STALE
...

monitor – watch for netlink messages.

manpages:

ip.man.txt

Links:

http://dwaves.de/2013/03/19/wie-funktioniert-arp-how-does-arp-work-specs-definition-specification/

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