What Is an SPI Firewall? How Is It Different From a Regular Firewall?

Tyler Cross
Tyler Cross Senior Writer
Updated on: May 31, 2024
Fact Checked by Sam Boyd
Tyler Cross Tyler Cross
Updated on: May 31, 2024 Senior Writer

A Stateful Packet Inspection (SPI) firewall is an advanced firewall that contextually examines packets of data traveling through your network. For example, it detects the source and destination IP addresses, port numbers, and the state of the connection, such as whether it’s part of an established session or a new connection attempt.

Plus, SPI firewalls apply predefined rules to incoming and outgoing packets. These rules determine whether to permit or deny the traffic. Moreover, SPI firewalls can dynamically adjust their rules based on changes in network traffic and security policies.

A traditional firewall performs basic packet filtering based on static criteria, such as source and destination IP addresses, port numbers, and protocol types. It analyzes each packet individually without considering the context of the overall communication session.

That said, an SPI firewall doesn’t protect you from all online threats. It’s not 100% foolproof and can still leave you vulnerable to malware and other viruses. So, I recommend using one alongside a high-quality antivirus engine, which can detect and remove all malware.

I tested the top antivirus suites with SPI firewalls, and Norton is my favorite. It consistently detected every network intrusion I attempted. Since it has a 60-day money-back guarantee, you can ensure it works for you.

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What Does a Firewall Do?

The simplest explanation is that firewalls block unauthorized access to your network.

Whenever information travels through a network, it’s broken up into smaller and easier-to-process data packets. Before these data packets are allowed to pass through to your network, a firewall analyzes them for threats.

There are multiple types of firewalls that each handle this task in their own unique way, to varying degrees of success. I’ll explain each below.

How Does an SPI Firewall Work?

An SPI firewall works by creating a set of rules for data on your network. It then combs through your data packets and searches for network activity that breaks the established rules. It also makes sure that each packet comes from an authorized connection.

Unlike traditional firewalls, the SPI firewall then monitors the state of active connections (hence, stateful) for any changes that would violate the rules. This also means that SPI firewalls require slightly better hardware than traditional firewalls, but you should be able to use an SPI firewall with almost any computer.

It does more for your network than analyzing data packets, though. Whenever new information comes through your network, the SPI firewall puts it on a track to be monitored. All incoming and outgoing communication from the track is observed, so the firewall can block a program the moment it starts exhibiting unusual behaviors. This means that even if a rule isn’t violated, the firewall can still detect suspicious activity.

Since it looks at both the individual data packets and the connection states, an SPI firewall protects you from sneaky threats that wait to deploy as well as immediate concerns.

However, they aren’t perfect. An SPI firewall mostly relies on details gathered from the heading of each data packet. This means it isn’t performing a deep dive into each packet, so some threats hiding inside legitimate-looking data packets can slip by.

Despite that drawback, SPI firewalls are one of the most reliable options due to their ability to provide simple networks with a high-security baseline. These are also the most cost-effective firewalls and are often bundled with cybersecurity suites (for example, Norton’s firewall uses SPI).

SPI Firewalls vs. Other Types of Firewalls

One thing to remember is there are many different types of firewalls. While I recommend an SPI firewall for most users, you should consider your own needs and what would benefit your network environment the most.

Stateless Packet Inspection Firewall

A stateless packet inspection firewall uses strict rule-based filtering. These only examine the headings of data packets and compare them against the pre-defined rules, blocking any connections that violate said rules. An SPI firewall continuously monitors network communications from its active tracks.

This makes a stateless packing inspection firewall worse at detecting threats than an SPI, but a stateless firewall still has its advantages. For example, stateless firewalls work faster than SPIs and are much simpler to install and use.

You can install a stateless firewall, fiddle with its rules, and leave it running in the background as part of a multi-layered security setup (meaning you’re combining it with other tools like an antivirus) with minimal confusion.

Even with those positives, a stateless firewall is worse at detecting threats. That’s why many top cybersecurity companies have converted their firewalls to stateful (like Norton’s smart firewall with SPI capabilities).

Overall, I like the simplicity and compatibility stateless firewalls have with other security tools but it’s tough to recommend them over a SPI firewall.

DPI Firewall

A DPI (Deep Packet Inspection) firewall delves deeper into data packets than SPI firewalls do, making them better for environments that need highly detailed inspections. DPI firewalls are better at catching malware, viruses, and other troublesome threats than traditional firewalls and can even tell when illegitimate data is hiding in a legitimate-seeming package.

It sounds pretty similar to an SPI firewall, but I promise the boring technical differences are important. While both firewalls analyze data packets for threats, they do it in fundamentally different ways.

An SPI firewall mostly examines information in the header of data packets, with some capabilities to go deeper based on its established rules, while a DPI firewall deeply examines the data packets. This also means that a DPI firewall is much slower than an SPI at scanning network data.

DPIs are great for businesses since the finer control over applications lets companies enforce specific network policies, and the enhanced protection is better at catching highly complex problems.

That said, they aren’t always a great fit for the average household. DPI firewalls can be pretty expensive and require a lot of technical experience to get the most out of them. If you’re looking to protect your home network, I’d encourage you to check out one of the SPI firewalls included with Norton or Bitdefender.

Next-Gen Firewall

Next-generation firewalls (or NGFWs) offer the most advanced protection against malware, intrusions, exploits, and other types of network attacks but they’re expensive.

The biggest difference between an NGFW and an SPI firewall is that an NGFW can identify and control applications regardless of the port or protocol being used. This gives you significantly more control over your firewall rules, security policies, and data management without notable drawbacks.

These also include integrated intrusion prevention systems, which enhance the ability of your network to protect itself from exploits, plus deep packet inspection. Basically, an NGFW does everything an SPI firewall and a DPI firewall can do and better.

However, they’re the most expensive on the market and, like DPIs, typically require expertise to use properly. They can also have steep hardware requirements for someone trying to protect their home network. Make no mistake, NGFWs are better at catching threats, but for the average person who just needs a good firewall, they’re a bit much.

Quick Comparison Table

Firewall Pros Cons
SPI Firewall ✅ Great security baseline, fast network scans, low hardware requirements, easy to use, great for home networks. ❌Relies on packet headers, not the best for the nastiest threats.
Stateless Packet Inspection ✅ Fastest network scans, lowest hardware requirements, highly compatible with other software. ❌Worst security features, least detailed network scans.
DPI Firewall ✅ Higher security baseline, lots of control over features, good for businesses. ❌Expensive, requires technical expertise to use properly, slow network scans.
Next-Gen Firewall ✅ Best network security features, best content analysis, most granular control over your network. ❌Highest technical requirements, requires expertise to use properly, most expensive, slowest scans.

Where to Find an SPI Firewall

Finding an SPI firewall is pretty simple. If you want to find a good SPI firewall, Windows and MacOS both come with built-in SPI firewalls, and they’re pretty good. Unfortunately, Android developers do not include firewalls, so you’ll need to use a third-party firewall for your phone.

Plus, most router comes with a built-in SPI firewall, which means you should have two layers of network defenses, one for your computer and one for your overall network.

However, popular cybersecurity suites offer the best SPI firewalls. For Windows, I’d nudge you towards checking out Norton or Bitdefender, as their bundled SPI firewalls consistently outperformed Windows’s built-in firewall in my tests. If you’re using Mac, Intego’s NetBarrier improves on macOS’s firewall rather than replacing it completely. If you’re on Android, Norton, Bitdefender, and McAfee all offer effective firewalls.

If you don’t need the extra security features included in those bundles, you can also check out some good free standalone firewalls. These still offer an upgrade over your device’s built-in protection without breaking the bank.

Quick summary of the best antiviruses with firewalls in 2024:

  • 🥇 1. Norton 360 — Best antivirus with an SPI firewall (consistently catches 100% of network intrusions).
  • 🥈 2. Bitdefender Total Security — Lightweight antivirus-bundled firewall that works great on low-end devices.
  • 🥉 3. McAfee  — Very reliable antivirus-bundled SPI firewall (best for large households).
  • Bonus. Intego — Best Mac antivirus with a firewall that improves upon macOS’ built-in firewall. 

Editors' Note: Intego and this site are in the same ownership group.

Is an SPI Firewall Enough Protection in 2024?

No, even the best SPI firewall in the world isn’t enough to completely protect you. While preventing threats that try to enter your network is extremely important, it’s only one facet of a multi-layered approach to security. Here are some examples of threats a firewall can’t protect you from:

  • Internal threats. A firewall only monitors your incoming and outgoing network traffic; it does nothing against threats already inside your device. For example, if you accidentally download malware, a firewall won’t help you get rid of it. To remove threats already on your device, you need to download a good antivirus program like Norton and run a full scan. An antivirus will analyze all of your files and destroy any threat it finds.
  • Phishing scams. Phishing scams happen when threat actors manage to trick you into giving away your personal information or downloading a malicious file. To avoid these, you should install web protection tools with anti-phishing protection, which firewalls lack. These tools prevent you from opening malicious links and block phishing websites.
  • Data leaks. If your data is exposed in a data leak, you need identity theft protection and identity monitoring services like Norton LifeLock provides.
  • Malware from legitimate sources. While a Next-Gen or a DPI firewall can find malware hiding in legitimate sources, an SPI firewall struggles with this. To make up for that flaw, you should use an antivirus with effective real-time protection. That way, if malware slips past your firewall, your antivirus will block it.
  • Encrypted data packets. If the data packet itself is encrypted, an SPI firewall can’t read its contents. As a result, you could contract malware by accepting encrypted data packets onto your network. Many modern firewalls account for this by including deep packet inspection, but this isn’t always a guarantee of safety, either. Again, an antivirus with good real-time protection and a malware scanner is your best bet.

Frequently Asked Questions

Do I need an SPI firewall?

Yes, an SPI firewall is a vital component of a multi-layered security setup. SPI firewalls monitor your incoming and outgoing network information to prevent intrusions. Unlike traditional firewalls, which only use a strict set of rules to filter data packets, SPI firewalls monitor active connections for suspicious activity or changes. The stateful part of stateful packet inspection means that it keeps a constant eye on your network by monitoring active applications while inspecting new packets.

Does Windows use a built-in SPI firewall?

Windows has a built-in firewall with SPI capabilities through Microsoft Defender, but it isn’t strictly an SPI firewall. Microsoft Defender scans incoming and outgoing traffic and compares it against a list of established rules while maintaining some aspects of a stateful inspection.

Because of that, many SPI firewalls can be better than Microsoft Defender. For example, when tested against Norton’s smart firewall with SPI capabilities, Microsoft Defender simply falls short security-wise.

Do SPI firewalls have limitations?

Yes. SPI firewalls mostly focus on analyzing the contents of a data packet header rather than digging deeply into the data packets themselves. This means that threats that are buried deeply in the data packet or inside legitimate data packets can slip past an SPI firewall’s defenses.

Some firewalls dig deeply into data packets (DPI and Next-Gen firewalls specifically), but these are typically slow, require experience to use, and are much more expensive than your average SPI firewall. That said, it’s pretty rare for threats that require a DPI firewall to deal with to target households, so an SPI firewall should be more than enough to protect you.

Is having an SPI firewall enough to protect my device?

No, an SPI firewall isn’t enough to protect your device. A firewall is just one of many tools designed to protect you and it can’t account for every type of threat — you’re still vulnerable to data leaks, malware infections, phishing scams, and more. The only way to completely protect yourself is to use a multi-faceted cybersecurity suite.

For example, Norton comes with an SPI firewall to protect your network as well as an excellent antivirus engine, an unlimited data VPN to encrypt your network, a web protection extension to block phishing scams, and a range of other tools to keep yourself safe.

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About the Author
Tyler Cross
Tyler Cross
Senior Writer
Updated on: May 31, 2024

About the Author

Tyler is a writer at SafetyDetectives with a passion for researching all things tech and cybersecurity. Prior to joining the SafetyDetectives team, he worked with cybersecurity products hands-on for more than five years, including password managers, antiviruses, and VPNs and learned everything about their use cases and function. When he isn't working as a "SafetyDetective", he enjoys studying history, researching investment opportunities, writing novels, and playing Dungeons and Dragons with friends."