Why is a Circuit Level Firewall known as a Stateful Firewall?

Why is a Circuit Level Firewall known as a Stateful Firewall?


A circuit-level firewall is known as a stateful firewall because of the way that it works. It does not just deal with packets, but also maintains information about the connections between hosts. This means it can be used to block certain types of traffic without blocking all traffic. For example, you could block outgoing HTTP requests while allowing incoming HTTP requests to pass through the firewall.

A definition of a circuit-level firewall is that it can be set up to block or allow traffic based on certain rules. This means if someone sends an email that includes an attachment it will not be blocked because many emails have attachments. Circuit-level firewalls are also known as stateful firewalls because they remember the state of each connection.

What is a Circuit Level Firewall?

A Circuit Level Firewall is a firewall that inspects network traffic at each circuit in the connection. It was created by Paul Mockapetris, who also designed the Domain Name System (DNS).

“A circuit-level firewall works at the lower network layers, specifically the Network and Transport Layers. In this way, it is similar to a stateful packet inspection (SPI) firewall, which also monitors traffic at these levels. Circuit-level firewalls are useful for monitoring applications that use nonstandard ports.”

Why is a Circuit Level Firewall known as a Stateful Firewall?

A circuit-level firewall is known as a stateful firewall because it’s more intelligent than a packet filtering firewall. A packet-filtering firewall blocks traffic by examining packets of data coming in and out, while a stateful firewall not only does this but also keeps track of the sessions. The latter remember who was on which side of the connection based on what flows in or out. This provides an additional measure of security for the administrator.

Though it is not as common, an application-level firewall can also provide a stateful inspection. This type of device actually has the capability to understand specific applications and protocols that are running on top of IP connections or UDP session connections. These devices can even check the contents of files being sent over a connection. These devices operate much like circuit-level firewalls do, but are able to go beyond simple session tracking because they can look inside packets or sessions to verify that traffic conforms to the rules being enforced.

How does it differ from other firewalls?

The firewall is different from other firewalls in that it has the ability to maintain state information. This allows the firewall to drop separate sessions between internet hosts. The firewall does this by maintaining conversations. This does not mean that it has to have the capability of being configured on a per-conversation or even per-destination host basis.  The individual packets are tracked. This is known as stateful processing.

The firewall keeps track of which packets going to and from a host or network are part of an existing connection. It allows the return packets of that session through without checking against its ruleset.

As the packets are statefully processed, this prevents sessions such as denial of service attacks. This is where a machine will send out packets to another machine and overwhelm it with sessions that the targeted host’s resources will not be able to handle. When the firewall filters by state, it will drop these flood types of requests going to or from a targeted host.

What are the advantages of using this type of firewall?

The Layers of Protection Style of Firewall can give you an added layer of protection. One benefit is the prioritization of the traffic. It’s also good for organizations with multiple offices to have a central hub, which enables them to concentrate on one internet connection instead of several. Another benefit is that it helps to enforce security policies, defining rules that are in place, and making the enforcement more efficient.

An additional benefit is the ability to provide security for a Virtual Private Network (VPN), and if you’re using a DSL or cable internet connection, it’s good since they typically require only one network address.  It’s also beneficial in that it can be used to separate the inside network from the outside network. A drawback is if you are using multiple types of protocols, you may need to purchase ASICs or Application Specific Integrated Circuits, which are not cheap. It is best suited for an internal network with a limited number of users.

Who should use this type of firewall and why?

A circuit-level firewall is designed for the purpose of protecting private networks from unwanted access. They are typically used by companies with a private, internal computer system that they want to protect from outside threats. Circuit-level firewalls are also used to prevent direct access to the company’s network and data before passing that traffic through a VPN. These types of firewalls can detect and block any type of attempt to establish contact with the private network or any attempt to enter this network. It is also possible for these types of firewalls to provide access based on a “need to know” principle rather than an open-ended policy. In other words, only those people who need to access the private network can do so. In most cases, circuit-level firewalls have a single interface that is used for connection with the Internet through which all traffic is allowed onto the network.


A circuit-level firewall is a type of firewall that monitors and analyzes data packets in real-time. Due to the fact it operates on an even more granular level than packet filtering, this type of firewall can be considered stateful.  A packet-filtering firewall will only look at certain aspects of each data packet, whereas a circuit-level firewall actually looks deeper into the packets to establish whether or not they were part of another data stream or connection previously. For this reason, it is considered stateful.

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