struct sock->inet_sock convert question

in kernel linux-2.6.16.60-0.54.5 code: static int inet_create(struct socket *sock, int protocol){struct sock *sk;struct inet_sock *inet;.... sk = sk_alloc(PF_INET, GFP_KERNEL, answer_prot, 1); if (sk == NULL) goto out; inet = inet_sk(sk);inet->is_icsk = (INET_PROTOSW_ICSK & answer_flags) == INET_PROTOSW_ICSK; if (SOCK_RAW == sock->type) { inet->num = protocol; if (IPPROTO_RAW == protocol) inet->hdrincl = 1; } if (ipv4_config.no_pmtu_disc) inet->pmtudisc = IP_PMTUDISC_DONT; else inet->pmtudisc = IP_PMTUDISC_WANT; inet->id = 0;....} my question is in the red line , why can convert like that? thanks!