I'm not a system administrator for YCombinator, so I couldn't tell you, but I assure you I have the IPv4 IPs of all systems I maintain (personal and enterprise) memorized.

Not even by choice, mind you, but they naturally cement themselves in my mind over time as I work on systems because they're just that basic.

IPv6, on the other hand, I have literally one memorized (::1), likely because it's even shorter than a typical IPv4 address.