What IP does nginx use for ip_hash sticky sessions?

From:

http://wiki.nginx.org/HttpUpstreamModule#ip_hash

The key for the hash is the class-C network address of the client.

Also from:

nginx-0.8.53/src/http/modules/ngx_http_upstream_ip_hash_module.c:
     91 static ngx_int_t
     92 ngx_http_upstream_init_ip_hash_peer(ngx_http_request_t *r,
     93     ngx_http_upstream_srv_conf_t *us)
     94 {
...
    114     if (r->connection->sockaddr->sa_family == AF_INET) {
    115
    116         sin = (struct sockaddr_in *) r->connection->sockaddr;
    117         p = (u_char *) &sin->sin_addr.s_addr;
    118         iphp->addr[0] = p[0];
    119         iphp->addr[1] = p[1];
    120         iphp->addr[2] = p[2];
    121
    122     } else {
    123         iphp->addr[0] = 0;
    124         iphp->addr[1] = 0;
    125         iphp->addr[2] = 0;
    126     }
...
    164         for (i = 0; i < 3; i++) {
    165             hash = (hash * 113 + iphp->addr[i]) % 6271;
    166         }

Validating the documentation. It would be pretty easy to modify the code to include data like the XFF.

While this question is quite old and the answer is correct, after some digging to resolve my own load balancing problem I found that there is a newer option to make the client ip based on the X-Forwarded-For or X-Real-IP and when combined with the ip_hash directive it properly balances load using the user's actual IP as the hash.

http://nginx.org/en/docs/http/ngx_http_realip_module.html

    set_real_ip_from        127.0.0.1;  # nginx and varnish on other ports
    real_ip_header          X-Real-IP;  # or X-Forwarded-For
#   real_ip_recursive       on;         # doesn't work on nginx 1.0