In IIS 4.0, each Web site was bound to a different IP address. This meant that each site had its own socket, which was not shared with sites bound to other IP addresses. These sockets are created when each site starts, and they can consume significant amounts of nonpaged memory (RAM). This memory consumption limits the number of sites bound to IP addresses that can be created on a machine, because, after a certain number of sites are started, the machine is out of RAM.
For IIS 5.0, this process has been modified so that sites bound to different IP addresses—but sharing the same port number—share the same set of sockets. The end result is that more sites can be bound to an IP address on the same machine than in IIS 4.0. In IIS 5.0, these shared sockets are used flexibly among all of the started sites, thus reducing resource consumption.
This is now the default behavior for IIS 5.0. In general, this behavior should not be modified. However, for critical sites that require a dedicated socket, you can set the DisableSocketPooling to TRUE to revert back to the IIS 4.0 behavior. This change should be made at the site level only, so that other sites can continue to take advantage of the new socket pooling feature.