fping - A tool to quickly ping N number of hosts to determine
        their reachability without flooding the network.

             Roland J. Schemers III - Stanford University
                   schemers@Slapshot.Stanford.EDU

      fping is a ping(1) like program which uses the Internet Control
      Message Protocol (ICMP) echo request to determine if a host is
      up. fping is different from ping in that you can specify any
      number of hosts on the command line, or specify a file containing
      the lists of hosts to ping. Instead of trying one host until it
      timeouts or replies, fping will send out a ping packet and move
      on to the next host in a round-robin fashion. If a host replies,
      it is noted and removed from the list of hosts to check. If a host
      does not respond within a certain time limit and/or retry limit it 
      will be considered unreachable. 

Site
      Stanford University has a large TCP/IP network with over 12,000 
      assigned IP addresses and over 100 IP subnets.

Problem and Issues

      With a large a number of IP addresses in use, its becomes more and
      more time consuming to check on which IP addresses are actively
      in use, and which critical machines (routers, bridges, servers, etc)
      are reachable. One example is we have a program which goes through
      all of our routers arp caches looking for IP addresses that are in 
      use. After finding a list of IP addresses that aren't in any arp
      caches fping can then be used to see if these IP addresses really
      aren't being used, or are just behind the routers. Checking 2500
      hosts (99% of which are unreachable) via ping can take hours.
       
Solution
      
       fping was written to solve the problem of pinging N number of hosts
       in an efficient manner. By sending out pings in a round-robin fashion
       and checking on responses as they come in at random, a large number of 
       hosts can be checked at once. Checking 2500 hosts (5 packets per host, 
       10 msec between ping packets) takes under 3 minutes. Also, using fping
       to check 30 routers that are currently reachable takes about 420
       milliseconds (elapsed real time).

       Unlike ping, fping is meant to be used in scripts and its
       output is easy to parse.