Why use both anonymous enum and define macros for constants? [duplicate]

Question

Why netinet/in.h defines constants this way?

enum
{
    IPPROTO_IP = 0,    
#define IPPROTO_IP          IPPROTO_IP
    IPPROTO_HOPOPTS = 0,   
#define IPPROTO_HOPOPTS     IPPROTO_HOPOPTS
    IPPROTO_ICMP = 1,      
#define IPPROTO_ICMP        IPPROTO_ICMP
    IPPROTO_IGMP = 2,      
    ...
};

Answer 1

There's probably other code relying on #ifdef IPPROTO_IP etc.