Not sure which IP to use

[TheCandyman]

I am going to be switching ISP and so we get new IP numbers. In doing so i have to change my SMTP to point to my new IP address. The SMTP is outside my nework. This is what i have:

WAN: x.x.161.20/30
Provider Side: x.x.161.21
Customer Side: x.x.161.22

LAN: x.x.174.56/30
Router/Gateway: x.x.174.57
Useable IP's: x.x.174.58-62

I am using a T1 so the connection will use two IP addresses, customer side is my side of the T1, the provider is the ISP so i think this is my public IP. Just want another set of eyes to make sure.

 

[mpennac]

Based on the information you have given, it sounds like you are asking which address you should use for the MX record in your DNS domain. This record is used to point SMTP traffic to your SMTP server.

If this is the case, you would want to use the x.x.161.22 address in the DNS A record that associates an IP address with the DNS name of your mail server. Since the name of your mail server should not change, you will not have to change the MX record that specifies the server name.

If you have had this working in the past, you must already have the port mapping configured to pass all TCP port 25 traffic to the LAN side IP address of your SMTP server.

Here is how it works when it is all setup:

1) Someone wants to send e-mail to somebody@yahoo.com
2) The sending server looks up the MX record for yahoo.com
3) The MX record gives them the name of a mail server. In this case it is mx3.yahoo.com.
4) The sending server looks up the IP address for mx3.yahoo.com. In this case, it is 64.156.215.7.
5) The sending server establishes a TCP connection with TCP port 25 of the server 64.156.215.7.

Hope this is what you were looking for.

mpennac

http://www.netprospecialists.com

 

[ChrisAC]

This is wrong ..

LAN: x.x.174.56/30
Router/Gateway: x.x.174.57
Useable IP's: x.x.174.58-62

If your assigned IP range is x.x.174.56/30 then you only have x.x.174.57 and x.x.174.58 available for use, not 58 to 62.

If this is correct and x.x.57 is your firewall then either you will be port forwarding SMTP from this address to your server, or you will be doing a static NAT from x.x.174.58 to your mail server. Only you know which one you are doing. Of course, if this is actually a /29 then you have .57 on the router and .58 to .62 available.

x.x.161.20/30 is just the WAN link between your routers external port and the ISP. This isn't your allocated IP range if you have another /30 routed via it, it's just a numbered link.

Chris.



**********************
Chris Andrew, CCNA, CCSA
chris@iproute.co.uk
**********************