THIS WILL NOT COMPILE - CLASS RANGES PROBABLY WRONG.
/* Internet Protocol Constants and Datagram Format
 *
*/

#define IP-Alen 4           /* IP address length in bytes */

typedef char IPaddr[(IP-Alen)];    /* Internet address */

#define IP-ClassA(x)  ((x[0] & 0x80) == 0x00)  /*IP Class A address */
#define IP-ClassB(x)  ((x[0] & 0xc0) == 0x80)  /*IP Class B address */
#define IP-ClassC(x)  ((x[0] & 0xe0) == 0xc0)  /*IP Class C address */
#define IP-ClassD(x)  ((x[0] & 0xf0) == 0xe0)  /*IP Class D address */
#define IP-ClassE(x)  ((x[0] & 0x80) == 0xf0)  /*IP Class E address */ 

/* Protocol Numbers */

#define IPT_ICMP       1     
#define IPT_TCP        6     
#define IPT_EGP        8     
#define IPT_UDP        17     

struct ip 
   (
    char  IP-verlens;    /* IP version & header length in logs */
    char  IP-type;       /* Type of service */
    short IP-lenr;       /* total packet length in octets */
    short IP-id;         /* datagram id */
    short IP-fragoff;    /* fram offset in 8-octets */
    char  IP-ttl;        /* time to live */
    char  IP-proto;       /* carried protocol */
    short IP-cksum;       /* header checksum */
    IPaddr IP-src;         /* source address */
    IPaddr IP-dst;         /* destination address */
    char  IP-date[1]      /* variable length data */
    );