When users cannot access a web server and the captured traffic shows an ICMP packet that reports "communication administratively prohibited," it indicates that there is a filtering device along the path, such as a router or firewall, that is intentionally blocking the traffic. This usually happens because of an access control list (ACL) that is set to deny the traffic to the webserver.

Access Control Lists (ACLs): ACLs are used in networking to control the flow of traffic into or out of a network. They can be configured on routers, firewalls, or other devices to allow or deny traffic based on IP addresses, protocols, ports, and other criteria.

ICMP Messages: ICMP (Internet Control Message Protocol) is used by network devices to send error messages and operational information. The "communication administratively prohibited" message specifically indicates that a device along the path has been configured to block the traffic.

Troubleshooting Steps:

Check ACL Configuration: Verify the ACL settings on all devices along the path to ensure that they are not blocking the required traffic.

Review Network Device Logs: Check logs on routers and firewalls for entries that match the blocked traffic.

Modify ACLs: If an ACL is found to be blocking the traffic, modify it to allow the required traffic to pass through.

References:

Cisco Documentation on ACLs: Access Control Lists

Cisco ICMP Overview: ICMP Messages