# ARP address resolution protocal

###### tags:`OSI`,`ARP`

ref: https://www.youtube.com/watch?v=ArLgEC4bsps


ref https://www.youtube.com/watch?v=mqWEWye-8m8&ab_channel=HusseinNasser

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#### IP function


#### ARP : ip to mac


operation: 1 for request ,2 for reply
EA: ethernet addr == MAC addr
send from A to router

traget(router) mac is unknown then filled with 00:00:00:00..


hardware len:6 bytes / protocal len:4 bytes



if target mac match then receive or discard

router send back reply ; operation type: reply 2


ARP receive response and update ARP table


now the ethernet frame target MAC can be filled



arp table query

#### ARP work example

C send to A


try to find A MAC addr in C cache table but in vain


send broadcast query A MAC addr


A receive message and others discrad this query.
record C MAC addr in A ARP cache table.
prepare send back message to C.




C add A MAC addr to C's ARP cache table


cf. dynamic has timer to expire the map record of IP:mac, owing to the IP:MAC for dynamic type might change ex. change NIC(network interface card)

ARP work within single network, not cross networks