source: https://www.securityfocus.com/bid/3420/info

Microsoft Internet Explorer contains a security-setting feature that can be modified according to a user's preferences. These settings control what actions a web site can take on a user's system.

A vulnerability exists in Internet Explorer, which could allow a web site to be viewed in the Local Intranet Zone, rather than the Internet Zone. Thus, allowing content to be viewed with less-restrictive security settings.

Converting the IP address of the target web site into a dotless IP address, and submitting it, will cause Internet Explorer to view the web site in the Local Intranet zone.

* Microsoft Security Bulletin MS01-055 states that there is a new variant of this issue, although no technical details have been provided. A cumulative patch has been released and IE 5.5 users are encouraged to install it. 

Example:

An option in a basic authenticated site is to pass on a username (and/or
password) in the URL like this:

http://mike@msdn.microsoft.com

Another possibility is to convert an IP address into a dotless IP address;
such an address is also called a DWORD address (some proxy servers, routers
or web servers do not allow this).

http://msdn.microsoft.com - IP: 207.46.239.122

Convert this IP address to a DWORD address:

207 * 16777216 = 3472883712
46 * 65536 = 3014656
239 * 256 = 61184
122 * 1 = 122
------------------------------------------------ +
= 3475959674

This DWORD address can be used to visit the site like:

http://3475959674
If we combine the URL login option with the DWORD IP address we'll get the
following URL:

http://mike@3475959674

The browser still thinks we are in the internet zone as expected.

Now we change the @ sign to its ASCII equivalent (%40):


------------------------
http://mike%403475959674
------------------------