-- Address Resolution Protocol display and control
Dynamically maps between IP addresses and Ethernet addresses, via a cache.
arp -a ? (169.254.77.120) at (incomplete) on en1 [ethernet] rtr.germans (192.168.1.1) at 0:18:1:8f:23:ed on en1 ifscope [ethernet] ? (192.168.1.2) at 0:1b:21:a4:32:bd on en1 ifscope [ethernet] ? (192.168.1.5) at 50:e5:49:df:6d:6a on en1 ifscope [ethernet] ? (192.168.1.6) at 0:50:fc:9c:e0:18 on en1 ifscope [ethernet] dgermans-ipad.germans (192.168.1.7) at 18:20:32:9f:54:75 on en1 ifscope [ethernet] ? (192.168.1.8) at (incomplete) on en1 ifscope [ethernet] ? (192.168.1.100) at 0:1b:dd:28:fc:4d on en1 ifscope [ethernet] ? (192.168.1.101) at 0:1f:c4:ef:5d:f1 on en1 ifscope [ethernet]The DNS server is queried to display hostnames.
If response is slow it may be the DNS server is the problem.
|No queries to DNS server (much faster)|
When an interface requests a mapping for an address not in cache, that message is queued and
a broadcasts on the associated network requesting the address mapping is sent.
If a response is provided, the new mapping is cached and any pending message is transmitted.
If no response is recieved after several requests, the host is considered to be down for a short period (Default 20 seconds), and the
EHOSTDOWN error is reported for host, and
EHOSTUNREACH for a router.
The cache is stored in the system routing table as dynamically-created host routes.
The route to a directly-attached Ethernet network is installed as a cloning route (one with the RTF_CLONING flag set), causing routes to individual hosts on that network to be created on demand.
Routes time out periodically (Default 20 minutes after validated;). An entry for a host which is not responding is a reject route (one with the
RTF_REJECT flag set).
Entries may be added, deleted or changed with the arp utility.
Manually-added entries may be
temporary or permanent,
and may be
published, in which case THIS system will respond to ARP requests for that target host.
ARP watches passively for hosts impersonating the local host (i.e. a host which responds to an ARP mapping request for the local host's address).
EHOSTUNREACHrouter. is nresponsive!
duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x.
another host on the local network which responds to mapping requests for its own IP address with a different Ethernet address, generally indicating that two hosts are attempting to use the same IP address.