Converting a Hostname to an IP Address

Converting a Hostname to an IP Address

Problem

You have a string representation of a host (such as www.oreilly.com), and you need to obtain the IP address from this hostname.

Solution

Use the Dns.GetHostEntry method to get the IP addresses. In the following code, a hostname is provided to the GetHostEntry method, which returns an IPHostEntry from which a string of addresses can be constructed. If the hostname does not resolve, a SocketException stating "No such host is known" is thrown.

	using System;
	using System.Net;
	using System.Text;

	// …

	public static string HostName2IP(string hostname)
	{
	    // Resolve the hostname into an iphost entry using the Dns class.
	    IPHostEntry iphost = System.Net.Dns.GetHostEntry(hostname);
	    // Get all of the possible IP addresses for this hostname.
	    IPAddress[] addresses = iphost.AddressList;
	    // Make a text representation of the list.

	    StringBuilder addressList = new StringBuilder( );
	    // Get each IP address.
	    foreach(IPAddress address in addresses)
	    {
	        // Append it to the list.
	        addressList.AppendFormat("IP Address: {0};", address.ToString());
	    }
	    return addressList.ToString( );
	}

	// …

	// Writes "IP Address: 208.201.239.37;IP Address: 208.201.239.36;"
	Console.WriteLine(HostName2IP("www.oreilly.com"));

Discussion

An IPHostEntry can associate multiple IP addresses with a single hostname via the AddressList property. AddressList is an array of IPAddress objects, each of which holds a single IP address. Once the IPHostEntry is resolved, the AddressList can be looped over using foreach to create a string that shows all of the IP addresses for the given hostname. If the entry cannot be resolved, a SocketException is thrown.

See Also

See the "DNS Class," "IPHostEntry Class," and "IPAddress" topics in the MSDN documentation.