I am not able to understand how bytes in memory are being mapped into a struct. My machine is a little-endian x86_64. The code was compiled with gcc 4.7.0 from the Win64 mingw32-64 distribution for Win64. These are contents of the relevant memory fragment: ...450002cf9fe5000040115a9fc0a8fe... And this is the struct definition: typedef struct ip4 { unsigned int ihl :4; unsigned int version :4; uint8_t tos; uint16_t tot_len; uint16_t id; uint16_t frag_off; // flags=3 bits, offset=13 bits uint8_t ttl; uint8_t protocol; uint16_t check; uint32_t saddr; uint32_t daddr; /*The options start here. */ } ip4_t; When a pointer to such an structure (let it be *ip4) is initialized to the starting address of the above pasted memory region, this is what the debugger shows for the struct's fields: ip4: address=0x8da36ce ip4->ihl: address=0x8da36ce, value=0x5 ip4->version: address=0x8da36ce, value=0x4 ip4->tos: address=0x8da36d2, value=0x9f ip4->tot_len: address=0x8da36d4, value=0x0 ... I see how ihl and version are mapped: 4 bytes for a long integer, little-endian. But I don't understand how tos and tot_len are mapped; which bytes in memory correspond to each one of them. Thank you in advance.