What would you do if 2,000 login attempts hit your server in an hour, all from IP addresses you have never seen before?

You could block them one by one. You could scan access logs manually, cross-referencing each address against known threat lists. Or you could run an IP reputation check and know within seconds which of those addresses are already flagged as malicious by the global security community — and which are legitimate users who just forgot their password.

IP reputation checking is one of the most underused defensive tools available to sysadmins, security analysts, and anyone running internet-facing infrastructure. While most organizations have firewalls and intrusion detection systems, surprisingly few proactively check the reputation of addresses hitting their servers. According to the 2025 IBM X-Force Threat Intelligence Index, 30% of all incidents started with abuse of valid accounts — attacks that a reputation check on the source IPs could have flagged before they succeeded.

This guide covers how IP reputation works, what the different types of blacklists actually mean, how to run checks, and what to do when you find a problem — whether the suspicious IP is attacking you, or whether your IP ended up on a blacklist.

What IP Reputation Actually Means

Every IP address accumulates a reputation based on the traffic associated with it. Think of it like a credit score for network addresses. An IP that has only been observed sending legitimate traffic has a clean reputation. An IP that has been caught sending spam, participating in DDoS attacks, scanning for vulnerabilities, or distributing malware accumulates negative marks.

IP addresses earn bad reputations through several categories of abuse:

• Spam origination — sending unsolicited bulk email, either directly or through compromised mail servers
• Brute-force attacks — automated credential stuffing and password guessing against SSH, RDP, email, and web login forms
• Botnet participation — the IP belongs to an infected device that is part of a command-and-control network
• Malware distribution — hosting or redirecting to malicious payloads, phishing pages, or exploit kits
• Port scanning — systematically probing networks for open ports and vulnerable services
• DDoS participation — sending attack traffic as part of volumetric or application-layer floods

The critical thing to understand: IP reputation is not a binary clean/dirty flag. It is contextual. An IP might be listed on a spam blocklist but have zero history of port scanning. A hosting provider IP might be flagged for serving malware in 2024 but have been cleaned and reassigned since then. This is why checking reputation across multiple sources matters more than checking a single list.

How IP Reputation Databases Work

Reputation databases collect intelligence from multiple sources, cross-reference it, and assign risk indicators to IP addresses. The data pipeline looks roughly like this:

Honeypots and spam traps

Honeypots are deliberately exposed servers that look like vulnerable targets — open SSH ports, fake web applications, simulated mail servers. They exist solely to attract and log attacks. Any IP that connects to a honeypot is, by definition, probing or attacking, since there is no legitimate reason to reach these systems. Honeypot networks operated by organizations like Spamhaus, Shadowserver, and SANS collect millions of data points daily.

Spam traps work on the same principle for email. They are email addresses that were never used to sign up for anything. Any mail they receive is definitionally unsolicited. When a mail server sends to a spam trap, its IP gets flagged.

Threat intelligence sharing

ISPs, CERTs (Computer Emergency Response Teams), security vendors, and law enforcement share threat data through structured feeds. When a hosting provider takes down a botnet command-and-control server, the IP addresses of all the bots that connected to it get shared across the intelligence community. When a CERT identifies a phishing campaign, the sending IPs propagate to threat feeds within hours.

Crowdsourced reporting

Platforms like AbuseIPDB aggregate reports from sysadmins worldwide. When your server logs show an IP hammering your SSH port, you report it. When thousands of admins report the same IP, the confidence score climbs. AbuseIPDB processes over one million abuse reports daily, creating a real-time picture of which IPs are actively causing problems across the internet.

Types of IP Blacklists

Not all blacklists serve the same purpose, and confusing them leads to poor security decisions. Here are the main categories:

Spamhaus: The heavyweight

Spamhaus deserves specific mention because its blocklists are the most widely referenced in the world. The Spamhaus Block List (SBL) is used by the majority of email providers to filter incoming mail. Their Combined Spam Sources (CSS) list alone typically contains between 2 and 4 million entries, with 300,000 to 400,000 new listings added every 24 hours. The Policy Block List (PBL) is even larger, covering more than 1.4 billion IPv4 addresses — nearly 40% of the entire routable IPv4 space — but this is not a blacklist of abusive IPs. It lists IP ranges that ISPs have designated as end-user ranges that should not be directly sending SMTP traffic.

Understanding the difference matters. If an IP shows up on the PBL, it is probably a residential connection — not necessarily malicious. If it shows up on the SBL or XBL, something is actively wrong.

How to Check IP Reputation with InfoSniper

Running an IP reputation check involves querying multiple blocklists and threat feeds simultaneously. Doing this manually against each list would take significant time. Tools automate the process.

Step-by-step walkthrough

1. Go to InfoSniper's IP Reputation Checker — the tool accepts any IPv4 or IPv6 address.
2. Enter the IP address you want to investigate. If you leave it blank, the tool checks your own IP — useful for verifying your mail server or VPN exit node is clean.
3. Review the results — the tool queries multiple DNS-based blocklists and returns the status for each one. You will see a clear pass/fail indicator for every list checked.
4. Check the geolocation context — the results include country, ISP, and AS data from InfoSniper's IP lookup, giving you geographic and network context alongside the reputation data.
5. Cross-reference if needed — for IPs that show hits, use the WHOIS lookup to identify the network operator and find the correct abuse contact for reporting.

# Check if 192.0.2.1 is on Spamhaus ZEN (combined list)
# Reverse the IP octets, append the DNSBL zone
dig +short 1.2.0.192.zen.spamhaus.org

# If it returns 127.0.0.x, the IP is listed
# No response = not listed
# 127.0.0.2 = SBL (direct spam source)
# 127.0.0.3 = SBL CSS (spam operation)
# 127.0.0.4-7 = XBL (exploited host)
# 127.0.0.10-11 = PBL (policy block)

# Check reputation for multiple IPs programmatically
# Use InfoSniper's API with your key
curl "https://www.infosniper.net/json.php?k=YOUR_KEY&ip_address=203.0.113.42"

# For bulk analysis, use the Bulk IP Upload tool
# Upload up to 100 IPs at once: infosniper.net/bulk-upload/

For ongoing monitoring or high-volume checks, the InfoSniper API can be integrated into scripts and security workflows. The Bulk IP Upload tool handles batch checks of up to 100 addresses at once — useful when you need to triage a large set of IPs from server logs.

Reading the Results: What Blacklist Hits Mean

A reputation check returns a list of blocklists and whether the IP appears on each one. Interpreting the results requires understanding what each listing type actually means.

Use Cases: When IP Reputation Checks Matter

Email server management

This is the original and still most common use case. If your organization runs its own mail server, your sending IP's reputation directly determines whether your emails reach inboxes or spam folders. A poor reputation can result in up to 89% of emails going undelivered. Before sending any campaign, check your mail server's IP. After deliverability drops, check it again — a blacklisting might have happened without any notification.

Firewall and access control

When you observe suspicious traffic in server logs, a reputation check tells you whether the source is already known to the security community. If an IP brute-forcing your SSH has a 100% abuse confidence score on AbuseIPDB and appears on Spamhaus XBL, you can block the entire subnet with confidence. If it is a clean IP, you might be dealing with a legitimate user who misconfigured their client — a different response entirely.

Fraud prevention

E-commerce and financial platforms check the reputation of IPs associated with transactions. An order placed from an IP listed as a known proxy, Tor exit node, or botnet participant carries higher fraud risk than one from a clean residential IP. Combined with geolocation data from tools like InfoSniper, reputation checks add a layer to multi-factor fraud scoring.

Ad fraud detection

Digital advertising loses billions annually to fraudulent clicks from botnets. Ad platforms check clicking IPs against reputation databases to filter non-human traffic. An IP with a history of botnet participation generating ad clicks is almost certainly fraudulent, regardless of what the user agent string says.

Website security

Web application firewalls (WAFs) and rate limiters use IP reputation as an input signal. When an IP address with a known bad reputation starts making requests, the WAF can immediately apply stricter rules — tighter rate limits, CAPTCHA challenges, or outright blocking — before the IP has a chance to find a vulnerability. This is proactive defense rather than reactive.

What to Do If Your IP Is Blacklisted

Discovering that your own IP address is blacklisted is alarming but fixable. The process is methodical: identify the problem, fix the root cause, then request delisting.

Step 1: Confirm the listing

Run your IP through InfoSniper's IP Reputation Checker to see exactly which lists have flagged it. Note the specific lists — each has its own delisting process.

Step 2: Identify the root cause

Do not request delisting before you know why you were listed. Common causes include:

• Compromised server or CMS — a hacked WordPress installation or outdated web application sending spam
• Open relay — a misconfigured mail server that allows unauthenticated relaying
• Infected device on the network — a workstation with malware generating abusive traffic
• Shared hosting fallout — another site on your shared server triggered the listing
• Inherited from previous owner — you received a dynamically assigned IP that was previously used by a spammer

Step 3: Fix the underlying problem

Patch the vulnerability, remove the malware, close the open relay, or contact your hosting provider about the shared IP issue. Check your WHOIS data to make sure your abuse contact information is correct — blocklist operators sometimes attempt to notify IP holders before listing.

Step 4: Request delisting

Each blocklist has its own removal process:

Spamhaus:      https://check.spamhaus.org/  (self-service removal tool)
Barracuda:     https://barracudacentral.org/lookups  (removal request form)
SpamCop:       Automatic — listings expire after abuse stops
Sorbs:         Decommissioned June 2024 — no longer active
AbuseIPDB:     Reports expire over time; no manual delisting
CBL (XBL):     https://www.abuseat.org/lookup.cgi  (self-service)

Step 5: Monitor ongoing

After delisting, check your IP reputation weekly for the first month. Set up automated monitoring if your organization runs mail servers or critical internet-facing services. Many IP reputation problems recur because the root cause was only partially addressed.

IP Reputation in Automated Security Workflows

Manual reputation checks work for individual investigations, but production security operations need automation. Here is how IP reputation fits into larger security architectures.

SIEM integration

Security Information and Event Management platforms (Splunk, Elastic SIEM, Microsoft Sentinel) can enrich incoming log events with IP reputation data automatically. When a firewall log entry arrives, the SIEM queries a threat intelligence feed, appends the reputation score, and adjusts the alert priority accordingly. A failed login from a clean IP generates a low-priority alert. The same failed login from an IP with active blacklist hits generates a high-priority incident.

Automated firewall blocking

Tools like Fail2Ban watch server logs for patterns (repeated failed SSH logins, web scanning signatures) and automatically block offending IPs. Combining Fail2Ban with reputation feeds creates a two-layer defense: known bad IPs get blocked before they even attempt an attack, and new attackers get blocked after their first suspicious action.

Threat intelligence platforms

Platforms like MISP (Malware Information Sharing Platform) and AlienVault OTX aggregate and share threat indicators including malicious IPs. Organizations contributing to these platforms help build collective defense — when one organization detects an attacker, the IP gets shared to protect everyone in the community.

Limitations: When Reputation Data Misleads

IP reputation checking is a powerful signal, but it has real blind spots that matter in practice.

False positives from shared infrastructure

Carrier-grade NAT (CGNAT) means hundreds or thousands of users share the same public IP address. If one user on the NAT pool runs malware, the shared IP gets blacklisted, and every other user behind that NAT is collateral damage. Cloudflare has documented this problem extensively, noting that CGNAT is a likely unseen source of bias on the internet, with the effects most pronounced in developing regions where IPv4 exhaustion is most severe.

The same applies to shared hosting, cloud hosting, and VPN exit nodes. An IP associated with AWS, DigitalOcean, or a popular VPN provider may carry reputation baggage from previous or concurrent users that has nothing to do with the current traffic you are investigating.

Dynamic IP assignment

Residential ISPs reassign IP addresses regularly. The IP that was flagged for spam last week may now be assigned to someone who has never sent a malicious packet. Conversely, a clean IP today might have been a botnet node yesterday. Dynamic assignment means reputation data has a shelf life, and older listings become less reliable.

Stale data

Some blocklists are more aggressively maintained than others. Spamhaus updates every five minutes. Smaller, volunteer-run lists might update weekly or monthly — or not at all. The SORBS blocklist, once widely used, was decommissioned in June 2024 and no longer contains any active reputation data. If a tool still queries SORBS, the results are meaningless.

IPv6 coverage gaps

Most reputation data is concentrated on IPv4. IPv6 adoption continues to grow, but the vast address space makes it harder to build comprehensive reputation databases. A clean IPv6 result might simply mean no one has reported that address yet, not that it is safe.