IP Allowlisting and Blocklisting

IP allowlisting and blocklisting are access control methods that permit or deny traffic based on source IP addresses. They sound like mirror images, but they embody fundamentally different philosophies of trust.

Allowlisting says: guilty until proven innocent. Blocklisting says: innocent until proven guilty. Neither is wrong—they're answers to different questions.

Allowlisting: Default Deny

IP allowlisting explicitly defines which addresses are permitted, denying all others. Nothing gets through unless specifically approved.

This makes sense when you can enumerate your legitimate users. If only three office locations should access your management interface, allowlist those three IP ranges and block everything else. If your database should only talk to your application servers, allowlist those servers and reject the rest of the Internet.

The security is strong precisely because it's paranoid. Unknown threats can't reach you—they're blocked by default, regardless of how clever or novel the attack.

The cost is rigidity. Every legitimate source must be known in advance. When a new office opens or a partner's IP changes, someone must update the list or access fails.

Blocklisting: Default Allow

IP blocklisting explicitly defines which addresses are denied, permitting all others. Everything gets through unless specifically blocked.

This makes sense when you can't enumerate your legitimate users—which describes most public services. Your website serves the world. You can't predict every IP address that might legitimately visit.

Blocklists target known bad actors: IP addresses observed launching attacks, hosting malware, running botnets, or sending spam. Security services maintain these lists, aggregating data from honeypots, attack reports, and abuse databases. Your firewall consumes these feeds and blocks traffic from known-bad sources before it reaches your applications.

The flexibility comes at a cost: you only block what you know about. The attacker who hasn't been caught yet, the newly compromised server, the fresh botnet—they're not on the list yet.

The Dynamic IP Problem

Many Internet users have IP addresses that change. Your home IP today might be different tomorrow. This creates headaches for both approaches.

For allowlisting: the employee you approved yesterday gets blocked when their ISP assigns them a new address. You could allowlist their entire ISP's range, but that includes millions of other users—defeating the purpose.

For blocklisting: the attacker you blocked reconnects and gets a fresh IP. They're back, and your blocklist entry now points at some innocent person who inherited the old address.

IP-based access control works best when addresses are stable. It struggles with the residential Internet's constant churn.

The Shared IP Problem

Network Address Translation means multiple users often share a single public IP address. One corporate office might have thousands of employees behind one IP. One hotel might have hundreds of guests.

Blocklisting that shared IP creates collateral damage. One guest's malware-infected laptop triggers a block, and suddenly every guest loses access. One employee's mistake, and the whole company is locked out.

The attacker moves on. The innocent users call support.

This over-blocking risk makes aggressive blocklisting dangerous for public-facing services. You might block more legitimate users than attackers.

Allowlisting has the opposite dynamic—if the company's IP is trusted, all employees benefit. But this assumes everyone behind that IP deserves trust, which isn't always true.

Geographic Filtering

Geographic IP filtering blocks or allows entire countries based on where IP addresses are registered.

If your business operates only in the United States, you might blocklist all non-US IPs. The logic: attackers in blocked countries can't even attempt access, reducing your attack surface.

The problems are predictable. IP geolocation isn't perfectly accurate. Attackers use VPNs to appear in allowed countries. Your own employees traveling abroad suddenly can't access anything.

Geographic filtering works better for internal resources—admin panels, management interfaces—than for customer-facing services where legitimate users might be anywhere.

Managing Lists at Scale

Small organizations can maintain IP lists manually. At scale, this becomes impossible.

Automated threat intelligence feeds provide continuously updated blocklists. Security Information and Event Management (SIEM) systems can automatically block IPs based on detected attack patterns. Cloud services and CDNs offer managed IP filtering with global enforcement.

Blocking duration matters too. Permanent blocks suit known-bad infrastructure unlikely to become legitimate. Temporary blocks—expiring after hours or days—handle dynamic IPs better and reduce collateral damage as different users rotate through addresses. Escalating blocks increase duration with repeated offenses: first offense gets an hour, second gets a day, third gets a week.

Why Attackers Win This Game

IP-based access control has limits that sophisticated attackers exploit routinely.

VPNs and proxy services let attackers hide behind different IPs. Block one, they switch to another. Compromised legitimate systems provide attack sources with clean reputations—the IP isn't on blocklists because it belonged to a trusted server until yesterday.

Some attacks spoof source IPs entirely. In DDoS amplification attacks, the apparent source isn't the actual attacker. Blocking it accomplishes nothing except denying access to whoever actually owns that address.

Distributed attacks from botnets overwhelm IP-based defenses through sheer volume. You can't blocklist a million-node botnet fast enough.

IP Controls as One Layer

IP-based access control works best as part of defense in depth, not as the sole protection.

Authentication verifies identity beyond IP address—an allowed IP still requires proper credentials. Multi-factor authentication adds security even if credentials are compromised. Encryption protects data regardless of source reputation. Behavioral analysis detects malicious activity even from seemingly legitimate IPs. Application-layer controls enforce business logic that network-level checks can't understand.

IP filtering is a bouncer checking IDs at the door. It's useful, but it's not the only security the building needs.

Best Practices

Use allowlisting for administrative and backend access where legitimate sources are known. Use blocklisting for public-facing services to block known threats without impacting unknown users.

Combine IP controls with authentication—IP address should be one factor, not the only factor.

Review lists regularly. Remove obsolete entries. Add new legitimate sources. Incorporate current threat intelligence.

Monitor for false positives and provide a process for blocked users to report issues.

Document entries: why each IP is allowed or blocked, when it was added, who added it.

Use temporary blocking for dynamic IPs to avoid indefinite punishment of addresses that change hands.