What Is an IP Blacklist and How Does it Work?

An IP blacklist a key tool in combating cybercrime because it flags suspicious internet protocols.

However, false positives and false negatives complicate the process, so organizations and their IT security staff need to arm themselves with a realistic expectation of how they can best utilize IP blacklists.

Let’s look at what IP blacklisting is, the different types, the ways in which they work, and how they help organizational security practices.

What Is an IP Blacklist?

An IP blacklist is the result of adding one or more IPs to a list of addresses that are, or will be, rejected or blocked. Though risk thresholds vary from organization to organization, the listed IPs will generally have been flagged for their association with malicious behavior, such as fraud, crime, and the funding of terrorism. The system exists to help ensure that a person or organization’s network security is as safe as possible from cyberattacks originating from harmful IPs.

There are various approaches to IP blacklisting, and it’s not always easy to achieve an accurate system of filtering the good from the bad when it comes to internet protocols. Nevertheless, there’s intuitive software like SEON that helps you spot suspicious accounts – check out what it can do by entering a contact detail like an email address into the text bar below.


Next up, let’s take a look at the various types of IP blacklisting and delve further into some of the processes it involves.

Types of IP Blacklist

There are two main types of IP blacklists: IP-based and domain-based. Within those two types, there are numerous subtypes based on the approaches taken to achieve such IP blacklists.

IP-based IP blacklisting is when the reputation of the IP address becomes associated with dangerous or potentially fraudulent behavior, presenting a sufficient reason for all connections from that IP to be blacklisted. In such a case, the IP may be linked to scamming activity, hacking, or worse. 

Meanwhile, domain-based IP blacklisting is based on the nature and activities of specific domains and/or URLs. These, if found suspicious due to signs of phishing attempts, threatware distribution, and so on, can be traced to their associated IP. This then leads that IP itself to being blacklisted.

IP-based blacklisting is less wide-reaching than its domain-based counterpart. The former focuses on individual IP addresses, whereas the latter scrutinizes various areas of internet traffic and network security, such as users’ web content and DNS records, before determining whether the IP should be blacklisted.

Regardless of whether you use IP-based or domain-based IP blacklisting (or both), there are also subtypes of IP blacklisting. These are differentiated from each other based on the specific means through which they achieve their goal.

Let’s take a look at various IP blacklisting subtypes.

IP blacklist subtypesConditions that define subtypeIP-based, domain-based, or both?
AutomaticThe algorithms of dedicated IT security software deem the IP to be associated with suspicious activity, such as consistent failed login attempts, and blacklist it.IP-based
ManualIT security staff consider the IP to be a threat and manually type it into an IP blacklist.IP-based
URL-basedThe URL or webpages linked to the IP are deemed suspicious, owing to their keywords, geolocation, or other relevant factors, so the security software or security staff blacklist the IP.Domain-based
Reputation-basedThe IP address or its associated domain is deemed by software or humans to be associated with suspicious activity, such as historical instances of hacking. This leads to a bad reputation for the internet protocol. As a result, it is either automatically or manually entered into an IP blacklist.Both

As reflected by the above table, there are numerous ways in which IPs can be blacklisted. While many approaches to IP blacklisting will be holistic, the following questions help to determine which individual type and subtype of IP blacklisting applies:

  • Is software or humans carrying out the blacklisting?
  • Is it based on suspicious URLs or suspicious online reputations?
  • Are the blacklisted IP addresses based on the IPs’ intrinsic qualities or the domain activity associated with them?

Why Do IP Addresses Get Blacklisted?

IP checking software or IT security staff blacklist IP addresses when they consider factors associated with them, such as their users’ behavior online, to be suspicious. For example, reports of internet users being abusive online can lead to certain networks rejecting their internet protocol. 

IP addresses are blacklisted for cybersecurity purposes. They enter a blacklist when cybersecurity software algorithms and/or IT administrators inspect them and decide they are potentially unsafe and should be rejected.

core reasons for Blacklisted IPs to occur

IP addresses can get blacklisted when they’re either correctly deemed suspicious (meaning a true positive is made) or when they’re incorrectly misunderstood as suspicious (meaning a false positive is made). The fact that there can also be false negatives exacerbates the problem. False negatives are when a suspicious IP is incorrectly considered safe by IP checking software.

One major reason that false negatives can happen is down to attackers abusing proxies, such as data center proxies and mobile proxies. Proxies are gateways between internet-connected devices and the websites they visit. They are often used legitimately for privacy purposes. However, when abused by an attacker, proxies can trick the targeted website’s server into thinking the IP is different from what it actually is.

For example, an attacker from a geolocation with a high incidence of cybercrime may fear the suspicion their country could raise and so decide to use a mobile proxy. Such a proxy can trick IP blacklisting software into registering the internet protocol as being a legitimate source from a low-crime geolocation.

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When IP addresses are blacklisted – regardless of whether they’re subject to true positives or false positives – they only become so when they meet certain criteria. These are some of the many scenarios that lead to a positive result from an IP blacklist check:

  • IT administrators encounter an IP address associated with devices that have a bad reputation, which often becomes the case after reports of malicious activity are made to the IT team.
  • IP scanning software encounters an IP address that is associated with one or more suspicious factors, such as a geolocation with a high incidence of cybercrime and a webpage whose keywords are associated with spam.
  • The IP address is associated with a web user who has violated certain terms and conditions, such as the terms of service of the website that they’re using.

These lay out the three core reasons why an IP address may be blacklisted: It is associated with a bad reputation, it is associated with suspicious activity, or it outright violates certain legal conditions.

There are many actions and features related to IP addresses that could fall under these categories. For example, IP addresses may carry a bot attack risk, facilitate file sharing without copyright, be involved in cyberbullying, or be connected to virtual private network (VPN) abuse if the IP user wishes to hide their activities (e.g. when they access illegal streaming services).

That being said, if you feel your IP connection has been blacklisted incorrectly – for example, because of connecting via a VPN or Tor client – it is important to appeal against it. No software algorithms and IT support staff are infallible, after all. If you are a security-minded user connecting via anonymizing services, some security teams may falsely assume you are a risk.

How to Check if an IP Has Been Blacklisted

There are numerous ways to find out if an IP address is on a blacklist. They essentially boil down to checking through or consulting with the right sources of information.

You can check through one of the dedicated online services for aggregating blacklisted IPs, a network security tool, or command-line tools. Otherwise, if your outbound email or browsing access has been blocked, you can ask the relevant email service provider or web host for the status of the IP, as they may have access to this information.


The best course of action will be based on your circumstances and preferences. If you are an innocent user whose IP was incorrectly blacklisted, you can enter your IP into a blacklist checker website or query it with the host of the email provider or website – because you have nothing to hide!

On the other hand, malicious actors such as hackers may want as few people as possible to know that they have been rightfully blacklisted, so they will likely use more covert and technical means, such as command-line tools. These are software programs that allow users to input software commands through text. The inputs can lead to the software providing network diagnostics, such as the IP address reputation or the SMTP (Simple Mail Transfer Protocol), which is the protocol that allows emails to reach their destination.

Data such as this can help programming-savvy people gauge whether the IP has connectivity problems that may signify it’s been blacklisted.

Top Four Signs That an IP Has Been Blacklisted

There are many signs when an IP is on a blacklist. The obvious one is that the IP user has been locked out of using websites or sending emails. There are also indirect signs, such as your emails to the user being blocked. Understanding these can help you determine whether you’re dealing with a blacklisted IP.

Let’s start with the obvious: A blacklisted IP can result in a webpage that says, “Unable to access the page”. In more subtle instances, security professionals who are trying to contact or exchange data with users of blocked IP addresses may encounter one or more of the following problems:

  • slower network connectivity
  • warnings about illegitimate internet activity from antivirus software
  • reductions in search engine page results
  • higher instances of emails that are landing in spam folders or outright failing to reach their destination

On top of the above, an unfortunate consequence that can occur when you keep trying to deal with a blacklisted IP is that doing so can increase your likelihood of being blacklisted too. In the event of a major security incident, responding fraud teams could trace malicious traffic through your domain, which may lead them to consider your website a security problem.

This is why it’s best for you and other security professionals to not just do IP blacklist checks, but utilize internet best practices, too. Your company should implement a strong fraud investigation software solution, use secure passwords, and regularly back up online data. 

What Are the Pros of Using IP Blacklists?

The pros of using IP blacklists are mainly down to internet security and reputational advantages. Successful IP blacklist checks help ensure safer internet use and bolster people’s trust in the webpages and email addresses that have not been blocked.

By contributing to IP blacklists, your company can help reduce the likelihood of internet users encountering spam and phishing. Increasing the awareness of which sites and entities to watch out for not only shores up the defenses of your own website, it improves the overall safety and activeness of the digital economy by pointing users towards safe business portals and away from harmful ones.

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The other advantage is that simply by existing, IP blacklists can deter potential attackers, as the knowledge of potential IP checks may make cybercriminals feel like they’re being watched. As such, they’ll have to face an extra layer of preparation, so may think twice before they act.

Ultimately, IP blacklists exist to make visiting and publishing webpages, as well as receiving and sending emails, safer and more trustworthy.

What Are the Drawbacks of Using IP Blacklists?

IP blacklists are made with the best of intentions, but they are a double-edged sword. This is because they come with the risk of false positives or other moments of friction in the user experience that could lead to frustration and churn. 

Despite the fact that they are a necessity, the use of IP blacklists brings difficulties to both the people utilizing them and the people encountering them.

This is because it is complicated to maintain and update IP blacklists, whether they’re automated, manual, or both. Internet traffic will only increase with time, and finding a way to police suspicious IP users will always be labor-intensive and time-consuming.

This is especially the case when the blacklisting system leads to false positives, which then have to be queried and resolved.

On top of this, even when someone isn’t blacklisted, they may encounter problems because of the slowed browsing functionality that comes from webpages or email servers that run IP blacklist checks in the background.

Aside from the problem of false positives, there is also the issue of IP blacklists that provide users with a false sense of security when such systems produce false negatives. The unfortunate fact is that well-prepared attackers can bypass IP blacklists through IP spoofing, proxies, and other tricks. The result is that those people may receive undeserved trust from IT security staff and other internet users.

How SEON Can Help with IP Blacklists

SEON is equipped to counteract suspicious IP activity thanks to its software focusing on IP analysis and the assigning of an IP fraud score. As the below animation shows, SEON has a dedicated API for checking the details of any IP that you enter into its IP lookup tool.

As the above shows, you simply need to enter the IP address of interest into SEON’s New Manual Check window. Then you can know whether the internet protocol is flagged as suspicious, as well as its geolocation and whether it is associated with certain proxies, spam hosts, hacking attempts, and more.

Fraudsters or attempted fraudsters may find their IPs blacklisted when they return to the scene of the digital crime, forcing them to use an IP spoofing service or another location anonymizing service. However, once flagged as fraudulent, SEON customers can track these users across IPs and accounts through advanced device fingerprinting.

These highly unique identifiers usually don’t change when a multi-accounting abuser connects to your website with different usernames. As such, a returning fraudster who has assumed a new IP can still be discovered, meaning you can get their new connecting IP blacklisted as well.

With SEON, you can also choose to contribute to our database of blacklisted data points, including IPs. These blacklists inform the entirety of our customer network while remaining compliant with all data privacy laws. These sources of information are crucial in providing web users with an IP fraud score and equipping them with the knowledge of which IPs are both trustworthy and suspicious – and ultimately helping them keep their domain secure with just a few clicks of a mouse.

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Author avatar
Sam Holland

Sam is SEON's Fraud Content Writer. He has a background in writing and editing content for a range of tech and engineering publications which has led him to gain a strong interest in cyber security. At SEON, Sam enjoys writing about cutting-edge solutions to fraud attempts and cyber attacks, such as transaction monitoring and machine learning.