Fix Public IP Issue

RouterOS General
1

I am connecting to my ISP via PPPoE connection. I am using DSL modem as bridge and RB750GL to do the PPPoE dial. I have the /30 mask. Following is the Fix Public IP address I will receive when connect:

  1. 120.120.3.0 ← network address
  2. 120.120.3.1 ← gateway address
  3. 120.120.3.2 ← IP for my server
  4. 120.120.3.4 ← broadcast address

My setting is when connected, my RB750GL will receive the gateway address (121.120.3.1) at WAN port and also NAT this address for use in my Private address at LAN Port. All my office computer will be behind the NAT and use the Private IP Address. Therefore, The IP Address 120.120.3.2 is not use.

I have a server and set as follows:

IP Address: 120.120.3.2
Net Mask : 255.255.255.252
Gateway : 120.120.3.1

and I plug this server to the RB750GL LAN port. The server can access to Internet and the Computers at the Private IP range can access the server as well. The only problem is when I use the server to visit the network-tools.com, the IP show is 120.120.3.1 not 120.120.3.2. What should I do at the RB750GL to expose the server IP at the Internet?

2

If the IP traffic is leaving you with the source set to 120.120.3.2 then your ISP must be presenting it upstream as 120.120.3.1 - i.e. the address from their end of the link net. I suspect that only your ISP can change that.

3

I don’t think so. This is because

  1. If I set the Router IP as 120.120.3.1 and the NAT address as 120.120.3.2, all the machine will go out as 120.120.3.2.
  2. In fact, the ISP provided a Technicolor TG784n v3 router can do what I mentioned in my first post. I just like to use the RB750GL to replace the Technicolor router and keep it as spare.

Anyone out there can help? Thanks.

4

OK - I think I misread your original post and was confused by the “gateway IP description”.

What does the status tab for your PPPoE show when connected?

5

If connected, PPPoE IP address is 120.120.3.1. All the computer in my office will go through the NAT and connected to Internet using this 120.120.3.1 as the IP address. I connected a server to the LAN port of the RB750GL and the setting of the server as follows:

IP Address: 120.120.3.2
Subnet Mask: 255.255.255.252
Default gateway: 120.120.3.1

The server can connect to Internet, but the IP appear on the network-tools.com is 120.120.3.1 and not 120.120.3.2. Please help.

Note: I did add a static route 120.120.3.2 255.255.255.255 192.168.80.254. <---- RB750GL LAN port address.

6

OK - this is one of those messy/grey areas where different routers take different approaches.

The cleanest solution which will work with any router is:

  1. Give the server a private address.
  2. Use SRC NAT to ensure that the server’s outbound traffic originates with the correct public address.
  3. Use DST NAT to pass relevant inbound traffic arriving at the public address to the server’s private address.

If that doesn’t work with the services on the server then you can follow the path you are currently on. You probably have a masquerade rule on the outbound WAN interface - try excluding the server public IP from that masquerade rule. You may also need proxy-arp on your WAN interface then to get the router to respond to ARP requests for the server public IP.