If you are using PTCL as your ISP with ZXDSL831CII Modem and want to forward your external IP to one of your desired internal IP then here is a complete tutorial with HD screenshot that will surely help you and its personally tested. If you still have any quires, feel free to ask us.
1: First Open Your Browser.
2: Type 192.168.1.1 In Address Bar.
3: Type "admin" In UserName And "admin" In Password.
4: Then Click 'Advance > Lan > DHCP Server' And Click "View Reserved IP List" Button. A POPUP Will Appear Then Click "Add" Button To Add A New Entry.
5: Add Your Desired Internal IP Address And Your Server(PC) MAC Address.
6: Now Go To 'NAT > Virtual Server' And Click "Add" Button Here And Add Your Desired Data Here.
7: First Add A Customized Name To Your This Entry, Then Add Your Protocol, Then Add Your External Port(ON Your External IP) That You Want To Forward And Then The Internal Port(Your Server IP Port) Where You Want To Forward External Port. Add Internal IP(Your Server IP) Where You Have Internal Port.
8: Now Go To 'Firewall > IP Filtering' And Click "Add" In "Inbound traffic".
9: Add Your Protocol Then Source IP(Select All To Add) And Add Your Destination IP(Internal Server-PC IP) With DNS, Add Your Internal Server-PC Port In 'From' And 'To' And Done.
10: Now Your IP Forwarding Is Done. Open Your External IP With Added Port In Browser.