I have a 2820n running firmware 3.3.5.2_2471201. My internet connection has a static IP address.

In the UI section WAN->Internet Access->WAN1 there is a section labelled "IP Address from ISP". If I leave this set to Dynamic then everything is fine. If, however, I switch it to Fixed IP = Yes and put my static IP address in the field then most things carry on working (web browsing etc), BUT my smtp server stops working for incoming mail. Why would this be?

This has been a little bit of an issue because obviously I did not suspect anything was wrong because most things carried on working fine. It was only when I noticed that no mail was coming in that I searched my memory for a possible reason and the only thing that coincided timewise was changing this setting.

Is the dynamic IP you get the same as your static one ?

Yes, it is. Exactly the same.

If your ISP is always going to hand out the same IP to you, then there is no real harm in leaving IP address from ISP to dynamic.

However, if you still prefer to have the fixed IP details in there then I suggest the following:

1) temporarily set WAN1 to dynamic

2) look under System Maintenance >> System Status

scribble down the ISP allocated

a) IP Address
b) Default Gateway
c) Primary DNS
d) Secondary DNS

3) you also need the subnet mask, which isn't shown with the status data (I think it would help if it were) but you can find this by looking at Diagnostics >> Routing Table

locate the line with a C and WAN1

e.g. C 22.33.44.1/ 255.255.252.0 is directly connected, WAN1

e) scribble down the subnet mask (the 255.*.*.* bit)

4) now you've got all the data, try setting WAN1 to fixed IP as you did before. My hunch is that a duff Subnet Mask value may have been causing the problem - so check this very carefully.

In other news, it might be worth switching to the 3.3.7 firmware for the overall improvements it has.

Regards, Neal.

I have decided to leave the setting at Dynamic but I am still interested to know what may have caused the problem that I experienced. There is no provision for setting the WAN1 subnet mask that I can find when specifying the Fixed IP address, so if this is causing the problem there seems to be a shortcoming in the UI in this respect.

When acquiring its IP address from the ISP on connection the subnet mask is set to 255.255.255.255 according to the routing table. Here is the routing table (anonymised). 22.33.44.55 refers to the router's IP address and 66.77.88.99 refers to the gateway address:

Key: C - connected, S - static, R - RIP, * - default, ~ - private
* 0.0.0.0/ 0.0.0.0 via 66.77.88.99, WAN1
C~ 10.0.0.0/ 255.255.255.0 is directly connected, LAN
S 22.33.44.55/ 255.255.255.255 via 22.33.44.55, WAN1
* 66.77.88.99/ 255.255.255.255 via 66.77.88.99, WAN1

The actual IP address which my router uses is the one on the line prefixed by an S. How does the router know that this is a static address when it was picked up on connection and the Fixed IP address field is not used? I did wonder whether the definition of "Static" might be not related to its assignation by the ISP but related instead to its setting within the LAN but that seems a bit far-fetched.

You suggest that the problem may be related to a duff subnet mask, presumably on the WAN1 connection, but I do not know how this should be set - there doesn't seem to be anything exposed in the UI for this purpose. How could the subnet mask have been duff, and how should one overcome that problem?

I note your comments about 3.3.7 but I have seen posts suggesting a number of issues with that release so I am holding fire at present.