Comparing IPv4 and IPv6

Fundamentals

tcpdump(1) of IPv4 TCP connection (loopback)
tcpdump(1) of IPv6 TCP connection (loopback)

C Language Programming

IPv4 socket structures
IPv6 socket structures

IPv4 Name Resolution
IPv6 Name Resolution

IPv4 packet header

 0                   1                   2                   3   
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|  IHL  |Type of Service|          Total Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Identification        |Flags|      Fragment Offset    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Time to Live |    Protocol   |         Header Checksum       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Source Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Destination Address                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Options                    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 0                   1                   2                   3   
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class |           Flow Label                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        |  Next Header  |   Hop Limit   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                         Source Address                        +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                      Destination Address                      +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

tcpdump(1) of IPv4 TCP connection

listening on lo0, link-type NULL (BSD loopback), capture size 96 bytes
11:43:30.141126 IP 127.0.0.1.14245 > 127.0.0.1.23:    Flags [S],  seq 1899213868, win 65535, options [mss 16344,nop,wscale 3,sackOK,TS val 7533732 ecr 0], length 0
11:43:30.141217 IP 127.0.0.1.23    > 127.0.0.1.14245: Flags [S.], seq 3809265911, ack 1899213869, win 65535, options [mss 16344,nop,wscale 3,sackOK,TS val 3742840880 ecr 7533732], length 0
11:43:30.141259 IP 127.0.0.1.14245 > 127.0.0.1.23:    Flags [.],                  ack 1, win 8960, options [nop,nop,TS val 7533732 ecr 3742840880], length 0

...
11:49:41.008987 IP 127.0.0.1.14245 > 127.0.0.1.23:    Flags [F.], seq 173,        ack 107, win 8960, options [nop,nop,TS val 7904657 ecr 3742841014], length 0
11:49:41.009075 IP 127.0.0.1.23    > 127.0.0.1.14245: Flags [.],                  ack 174, win 8960, options [nop,nop,TS val 3743211805 ecr 7904657], length 0
11:49:41.012735 IP 127.0.0.1.23    > 127.0.0.1.14245: Flags [F.], seq 107,        ack 174, win 8960, options [nop,nop,TS val 3743211808 ecr 7904657], length 0
11:49:41.012896 IP 127.0.0.1.14245 > 127.0.0.1.23:    Flags [.],                  ack 108, win 8959, options [nop,nop,TS val 7904661 ecr 3743211808], length 0

tcpdump(1) of IPv6 TCP connection

11:56:47.750525 IP6 ::1.49816      > ::1.23:          Flags [S],  seq 4281000056, win 65535, options [mss 16324,nop,wscale 3,sackOK,TS val 8331463 ecr 0], length 0
11:56:47.750636 IP6 ::1.23         > ::1.49816:       Flags [S.], seq 3300695537, ack 4281000057, win 65535, options [mss 16324,nop,wscale 3,sackOK,TS val 220526000 ecr 8331463], length 0
11:56:47.750697 IP6 ::1.49816      > ::1.23:          Flags [.],                  ack 1, win 8960, options [nop,nop,TS val 8331463 ecr 220526000], length 0

...
11:57:10.519518 IP6 ::1.49816      > ::1.23:          Flags [F.], seq 173,        ack 107, win 8960, options [nop,nop,TS val 8354236 ecr 220526134], length 0
11:57:10.519632 IP6 ::1.23         > ::1.49816:       Flags [.],                  ack 174, win 8960, options [nop,nop,TS val 220548773 ecr 8354236], length 0
11:57:10.523294 IP6 ::1.23         > ::1.49816:       Flags [F.], seq 107,        ack 174, win 8960, options [nop,nop,TS val 220548776 ecr 8354236], length 0
11:57:10.523486 IP6 ::1.49816      > ::1.23:          Flags [.],                  ack 108, win 8959, options [nop,nop,TS val 8354240 ecr 220548776], length 0

From FreeBSD 8.0:
struct ip {
#if BYTE_ORDER == LITTLE_ENDIAN
        u_int   ip_hl:4,                /* header length */
                ip_v:4;                 /* version */
#endif
#if BYTE_ORDER == BIG_ENDIAN
        u_int   ip_v:4,                 /* version */
                ip_hl:4;                /* header length */
#endif
        u_char  ip_tos;                 /* type of service */
        u_short ip_len;                 /* total length */
        u_short ip_id;                  /* identification */
        u_short ip_off;                 /* fragment offset field */
#define IP_RF 0x8000                    /* reserved fragment flag */
#define IP_DF 0x4000                    /* dont fragment flag */
#define IP_MF 0x2000                    /* more fragments flag */
#define IP_OFFMASK 0x1fff               /* mask for fragmenting bits */
        u_char  ip_ttl;                 /* time to live */
        u_char  ip_p;                   /* protocol */
        u_short ip_sum;                 /* checksum */
        struct  in_addr ip_src,ip_dst;  /* source and dest address */
} __packed __aligned(4);

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From FreeBSD 8.0:
struct ip6_hdr {
        union {
                struct ip6_hdrctl {
                        u_int32_t ip6_un1_flow; /* 20 bits of flow-ID */
                        u_int16_t ip6_un1_plen; /* payload length */
                        u_int8_t  ip6_un1_nxt;  /* next header */
                        u_int8_t  ip6_un1_hlim; /* hop limit */
                } ip6_un1;
                u_int8_t ip6_un2_vfc;   /* 4 bits version, top 4 bits class */
        } ip6_ctlun;
        struct in6_addr ip6_src;        /* source address */
        struct in6_addr ip6_dst;        /* destination address */
} __packed;










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struct sockaddr_in {
        uint8_t sin_len;
        sa_family_t     sin_family;
        in_port_t       sin_port;
        struct  in_addr sin_addr;
        char    sin_zero[8];
};

struct in_addr {
        in_addr_t s_addr;
};

typedef uint32_t        in_addr_t;

struct sockaddr_in6 {
        uint8_t         sin6_len;       /* length of this struct */
        sa_family_t     sin6_family;    /* AF_INET6 */
        in_port_t       sin6_port;      /* Transport layer port # */
        uint32_t        sin6_flowinfo;  /* IP6 flow information */
        struct in6_addr sin6_addr;      /* IP6 address */
        uint32_t        sin6_scope_id;  /* scope zone index */
};

struct in6_addr {
        union {
                uint8_t         __u6_addr8[16];
                uint16_t        __u6_addr16[8];
                uint32_t        __u6_addr32[4];
        } __u6_addr;                    /* 128-bit IP6 address */
};

struct hostent {
        char    *h_name;        /* official name of host */
        char    **h_aliases;    /* alias list */
        int     h_addrtype;     /* host address type */
        int     h_length;       /* length of address */
        char    **h_addr_list;  /* list of addresses from name server */
#define h_addr  h_addr_list[0]  /* address, for backward compatibility */
};

struct sockaddr {
        unsigned char   sa_len;         /* total length */
        sa_family_t     sa_family;      /* address family */
        char            sa_data[14];    /* actually longer; address value */
};

struct addrinfo {
        int     ai_flags;       /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */
        int     ai_family;      /* PF_xxx */
        int     ai_socktype;    /* SOCK_xxx */
        int     ai_protocol;    /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
        socklen_t ai_addrlen;   /* length of ai_addr */
        char    *ai_canonname;  /* canonical name for hostname */
        struct  sockaddr *ai_addr;      /* binary address */
        struct  addrinfo *ai_next;      /* next structure in linked list */
};

#include 

struct hostent * gethostbyname(const char *name);
struct hostent * gethostbyaddr(const void *addr, socklen_t len, int type);

#include 
#include 
#include 

void         freeaddrinfo(struct addrinfo *ai);
int          getaddrinfo(const char *hostname, const char *servname,
         const struct addrinfo *hints, struct addrinfo **res);
const char * gai_strerror(int ecode);