'ip inspect' command verifies if the HTTP packets are in response to the communication initiated from the router. If it doesn't match, the packets will be dropped.
The Cisco IOS Firewall performs inspections for TCP and UDP traffic. For example, TCP inspections include Telnet traffic (port 23, by default) as well as all other applications on TCP such as Hypertext Transfer Protocol (HTTP), e-mail, instant message (IM) chatter, and so on. Therefore, there is no easy way to inspect Telnet traffic alone and deny all other TCP traffic.
The Granular Protocol Inspection feature allows you to specify TCP or UDP ports using the PAM table. As a result, the Cisco IOS Firewall can restrict traffic inspections to specific applications, thereby permitting a higher degree of granularity in selecting which protocols are to be permitted and denied
Also refer to http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a0080455ad3.html
