Private, Reserved, and Public IP Ranges

An SSRF filter has to know which addresses are internal. This is the map: RFC 1918 private space, loopback, link-local, carrier-grade NAT, the documentation ranges, and everything else that is public and routable. Knowing the ranges is what turns a raw address into a safe-or-not decision.

Not every IP address is reachable from the public internet, and an check exists to tell the two apart. These are the ranges that matter.

Private address space (RFC 1918)

Three blocks are set aside for private networks and are never routed on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Inside a corporate network or a cloud (virtual private cloud), these are where internal services live, so an SSRF that reaches them can talk to databases, caches, and admin panels.

Loopback and link-local

127.0.0.0/8 is loopback: it always means "this machine," so http://127.0.0.1/ or http://localhost/ targets services bound to the server itself. 169.254.0.0/16 is link-local (RFC 3927), assigned automatically on a segment. The single address 169.254.169.254 inside that block is the cloud metadata endpoint, which is why link-local gets special attention.

Carrier-grade NAT and documentation ranges

100.64.0.0/10 (RFC 6598) is shared address space used by ISPs for -grade ; it is not public but not classic private space either, so it deserves a cautious middle classification. The TEST-NET blocks 192.0.2.0/24, 198.51.100.0/24, and 203.0.113.0/24 (RFC 5737) exist only for documentation and should never appear as a real destination.

Everything else is public

An address that falls in none of the reserved blocks is public and routable. Public destinations still carry some SSRF risk, since an attacker can point the server at a host they control, but they cannot reach your internal network. That is why a classifier reports a level of risk rather than a simple yes or no: the category of the destination decides how much to worry.

The ranges that get forgotten

RFC 1918 is the part everybody remembers, and a check that stops there has holes in specific, predictable places.

Loopback (127.0.0.0/8 — the whole /8, not one address) and link-local (169.254.0.0/16, which is where cloud metadata lives). Carrier-grade NAT (100.64.0.0/10) is neither private nor safely public. 0.0.0.0/8, which several stacks treat as "this host". And the IPv6 equivalents: ::1, unique-local fc00::/7, link-local fe80::/10, plus IPv4-mapped addresses like ::ffff:127.0.0.1 that carry a forbidden IPv4 address inside a permitted-looking IPv6 one.

This is the argument for using the platform's own classification — is_private, is_loopback, is_link_local — rather than a hand-written list of blocks. The library maintainers have already met the cases you have not, and their list is updated by people whose job it is.