C# 4 and .NET 4

Utility Classes ❘ 655
In this example, the byte integer address is assigned a binary representation of the IP address, and the
string ipString is assigned the text “234.56.78.9“.
IPAddress also provides a number of constant static fields to return special addresses. For example, the
Loopback address allows a machine to send messages to itself, whereas the Broadcast address allows
multicasting to the local network:
// The following line will set loopback to "127.0.0.1".
// the loopback address indicates the local host.
string loopback = IPAddress.Loopback.ToString();
// The following line will set broadcast address to "255.255.255.255".
// the broadcast address is used to send a message to all machines on
// the local network.
string broadcast = IPAddress.Broadcast.ToString();
IPHostEntry
The IPHostEntry class encapsulates information relating to a particular host computer. This class
makes the host name available via the HostName property (which returns a string), and the AddressList
property returns an array of IPAddress objects. You are going to use the IPHostEntry class in the next
example: DNSLookupResolver.
Dns
The Dns class is able to communicate with your default DNS server to retrieve IP addresses. The two
important (static) methods are Resolve(), which uses the DNS server to obtain the details of a host with a
given host name, and GetHostByAddress(), which also returns details of the host but this time using the
IP address. Both methods return an IPHostEntry object:
IPHostEntry wroxHost = Dns.Resolve("www.wrox.com");
IPHostEntry wroxHostCopy = Dns.GetHostByAddress("208.215.179.178");
In this code, both IPHostEntry objects will contain details of the Wrox.com servers.
The Dns class differs from the IPAddress and IPHostEntry classes because it has the ability to actually
communicate with servers to obtain information. In contrast, IPAddress and IPHostEntry are more along
the lines of simple data structures with convenient properties to allow access to the underlying data.
The Dnslookup example
The DNS and IP-related classes are illustrated with an example that
looks up DNS names: DnsLookup (see Figure 24-10).
This sample application simply invites the user to type in a DNS name
using the main text box. When the user clicks the Resolve button, the
sample uses the Dns.Resolve() method to retrieve an IPHostEntry
reference and display the host name and IP addresses. Note how the host
name displayed may be different from the name typed in. This can occur
if one DNS name (www.microsoft.com) simply acts as a proxy for
another DNS name (lb1.www.ms.akadns.net).
The DnsLookup application is a standard C# Windows application. The
controls are added as shown in Figure 24-10, giving them the names
txtBoxInput, btnResolve, txtBoxHostName, and listBoxIPs,
respectively. Then, you simply add the following method to the Form1
class as the event handler for the buttonResolve Click event:
void btnResolve_Click (object sender, EventArgs e)
{
try
{
IPHostEntry iphost = Dns.GetHostEntry(txtBoxInput.Text);
figure 24-10
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