Aneffective way to bypass firewall is using IP address spoofing where users gainan unauthorised access to a computer or network then deceiving the IP addressof the machine by making it seem like the message sent came from a trustedmachine.
It also includes techniques that attackers commonly use to performmalicious activities such as Denial of Service (DOS), Dedicated Denial ofService (DDoS) and Man in the Middle (MitM). To counter this, a good understandingof how and why an attacker would use the spoofing attack would help, inaddition to performing regular assessment of the firewall and its configurationagainst best range of configuration practices.  Sourcerouting is another effective method to bypass firewall. The sender of thepackets can designate the route that the packet would take in the network andwhen the packets have travelled among the nodes within the network, each routerwill perform checks on the IP address of the destination in the packets,forwarding them to the next node.
Using this technique, the sender makes some,or most, of the decisions on the router. An attacker can change the decisionsbut a way to prevent it is through record route, where the route of an IPpacket is traced, strict source routing where the sender specifies the exactroute for the packet to take or through loose source routing where the set ofrouters to visit is recorded using a source routing option.  Attackerscan bypass firewall using tiny fragments. This is when the user uses the IP fragmentationin order to create small fragments, where the TCP header information is forced toseparate into packet fragments. The purpose is to bypass the filtering rulesthat depends on the TCP header information, in hope that only the firstfragment is examined by the filtering router, and as a result, pass theremaining fragments through. To prevent this attack is by blocking the fragmentsthat holds less than a complete header.