401 - Unauthorized, Access Denied, 401 Page., in simple language You do not have permission to view this directory or page using the credentials that you supplied. Attacker can mask identity and browse. IP Spoofing prevention, IP Spoofing detection etc. are quite bigger super-specialities in network security. IP Spoofing can have apparent resemblance with Session Hijacking. Obviously, at worse your way can be used to bring a Ddos attack, SYN flood or DNS Amplification Attack.
You can read the list of HTTP Error Codes.