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Source: Linux.com - Posted by Pax Dickinson
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Today almost every organization employs firewalls for enhanced security. Firewalls can be set up in such a way that Internet Control Message Protocol (ICMP) requests are blocked, which means that traditional pings do not work. Setting a firewall to block ICMP requests is based on the theory that if a would-be hacker cannot "see" the target, he may not attack the host.
This makes system and network administration more difficult. A failed ping is no longer a valid test -- the user may have enabled a firewall that is blocking the ping, but the host may still be up. Before a network administrator can accurately determine if a host is down, the user needs to turn off all firewall applications -- or at least turn off any rules blocking ICMP -- which is probably asking too much of the average user.
If traditional ICMP-based pings are no longer reliable unless you know in advance that there is no firewall blocking ICMP echo requests, what other options exist? One option is an Address Resolution Protocol (ARP) based ping using the arping utility.
To know why ARP pings are virtually guaranteed to work while ICMP pings may not, one should understand the importance of ARP in networking. ARP is used by hosts on a network to resolve IP addresses into Media Access Control (MAC) addresses, which can be interpreted as a network interface's unique serial number. Hosts on an Ethernet network use MAC addresses rather than IP addresses to communicate.
Read this full article at Linux.com
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