uses
  SysUtils,  // for PByteArray
  Winsock;   // for htons()
 
const
  IP_DF = $4000;  // don't fragment flag
  IP_MF = $2000;  // more fragments flag
 
type
  in_addr = record ... end;  // you must fill this in
 
  iphdr_ptr = ^iphdr;
  iphdr = packed record
    ip_hl__ip_v: byte;      // header length, version: four bits each
    ip_tos:      byte;      // type of service
    ip_len:      shortint;  // total length
    ip_id:       word;      // identification
    ip_off:      shortint;  // fragment offset field
    ip_ttl:      byte;      // time to live
    ip_p:        byte;      // protocol
    ip_sum:      word;      // checksum
    ip_src:      in_addr;   // source address
    ip_dst:      in_addr    // destination address
    end;
 
procedure RecalculateIPChecksum( pIpHeader: iphdr_ptr );
  var
    sum:    longword;
    i:      longword;
    buff:   PByteArray;  // see note 1
  begin
  sum := 0;
  i   := 0;
 
  // Initialize checksum to zero
  pIpHeader^.ip_sum := 0;
  buff := pIpHeader;
 
  // Calculate IP header checksum
  while i < (pIpHeader^.ip_hl__ip_v and $F) *sizeof( longword ) do  // see note 2
    begin
    inc( sum, (buff[ i ] shl 8) +buff[ i +1 ] );
    inc( i, 2 )
    end;
 
  // Keep only the last 16 bits of the 32 bit calculated sum and add the carries
  while (sum shr 16) <> 0 do
    sum := (sum and $FFFF) +(sum shr 16);
 
  // Take the one's complement of sum
  sum := sum xor $FFFFFFFF;
 
  // ...and store it in network order
  pIpHeader^.ip_sum := htons( sum )
  end;