The traditional means of establishing the existence (on a network) of a host is, as mentioned elsewhere in this thread, to use one of the forms of ping.

However, be warned that, when pinging hosts/addresses on the inter-web, the results may not provide a definitive answer since an ever increasing number of sites/networks have firewall rules configured to prevent, by one means, or another, the forwarding of ping requests &/or replies - thus a target host that appears to be failing to respond to a ping, may in fact exist and be on-line but the traffic is disallowed.