uses 
 WinSock; 

function LocalIP: string; 
type 
 TaPInAddr = array[0..10] of PInAddr; 
 PaPInAddr = ^TaPInAddr; 
var 
 phe: PHostEnt; 
 pptr: PaPInAddr; 
 Buffer: array[0..63] of Char; 
 I: Integer; 
 GInitData: TWSAData; 
begin 
 WSAStartup($101, GInitData); 
 Result := ''; 
 GetHostName(Buffer, SizeOf(Buffer)); 
 phe := GetHostByName(buffer); 
 if phe = nil then Exit; 
 pPtr := PaPInAddr(phe^.h_addr_list); 
 I := 0; 
 while pPtr^[I] <> nil do 
 begin 
 Result := inet_ntoa(pptr^[I]^); 
 Inc(I); 
 end; 
 WSACleanup; 
end; 

function IPAddrToName(IPAddr: string): string; 
var 
 SockAddrIn: TSockAddrIn; 
 HostEnt: PHostEnt; 
 WSAData: TWSAData; 
begin 
 WSAStartup($101, WSAData); 
 SockAddrIn.sin_addr.s_addr := inet_addr(PChar(IPAddr)); 
 HostEnt := gethostbyaddr(@SockAddrIn.sin_addr.S_addr, 4, AF_INET); 
 if HostEnt <> nil then 
 Result := StrPas(Hostent^.h_name) 
 else 
 Result := ''; 
end; 

procedure TForm1.Button1Click(Sender: TObject); 
begin 
 ShowMessage(IPAddrToName(LocalIp)); 
end; 

{**************************************} 

// Function to get the IP Address from a Host 

function GetIPFromHost(const HostName: string): string; 
type 
 TaPInAddr = array[0..10] of PInAddr; 
 PaPInAddr = ^TaPInAddr; 
var 
 phe: PHostEnt; 
 pptr: PaPInAddr; 
 i: Integer; 
 GInitData: TWSAData; 
begin 
 WSAStartup($101, GInitData); 
 Result := ''; 
 phe := GetHostByName(PChar(HostName)); 
 if phe = nil then Exit; 
 pPtr := PaPInAddr(phe^.h_addr_list); 
 i := 0; 
 while pPtr^[i] <> nil do 
 begin 
 Result := inet_ntoa(pptr^[i]^); 
 Inc(i); 
 end; 
 WSACleanup; 
end;