Not every IP address is reachable from the public internet, and an SSRF 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 VPC, 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 carrier-grade NAT; 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.