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include/uapi/linux/in.h

Definitions of the Internet Protocol.

in_addr

in_addr stores an IP 4 internet address, i. e. 4 bytes (or 32 bits) in big endian order: 
struct in_addr {
	__be32	s_addr;
};

sockaddr_in

__be16 is a big endian unsigned int. 
struct sockaddr_in {
  __kernel_sa_family_t	sin_family;	/* Address family		*/
  __be16		sin_port;	/* Port number			*/
  struct in_addr	sin_addr;	/* Internet address		*/

  /* Pad to size of `struct sockaddr'. */
  unsigned char		__pad[__SOCK_SIZE__ - sizeof(short int) -
			sizeof(unsigned short int) - sizeof(struct in_addr)];
};
Compare sockaddr_in with sockaddr.
The _in in sockaddr_in likely stands for internet (hence the members sin_port and sin_addr where s possibly stands for socket).

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