The InetAddress class has two isReachable() methods that test whether a particular node is reachable from the current host (i.e., whether a network connection can be made). Connections can be blocked for many reasons, including firewalls, proxy servers, misbehaving routers, and broken cables, or simply because the remote host is not turned on when you try to connect.
public boolean isReachable(int timeout) throws IOException public boolean isReachable(NetworkInterface interface, int ttl, int timeout) throws IOException
These methods attempt to use traceroute (more specifically, ICMP echo requests) to find out if the specified address is reachable. If the host responds within timeout milliseconds, the methods return true; otherwise, they return false. An IOException will be thrown if there’s a network error. The second variant also lets you specify the local network interface the connection is made from and the “time-to-live” (the maximum number of network hops the connection will attempt before being discarded).