Number of IP Alias that can be configured
With the IP Alias feature, is WAN IP Alias automatically set to the primary public IP address assigned by the ISP, and what is the maximum number of auxiliary IP Alias addresses that can be entered?
WAN IP Alias is automatically set to whatever the primary public IP address assigned by the ISP. The 2820 allows a total of 8 IP Alias entries, thus giving you 7 additional IP's that can be configured
Restrictions on the Range of IP Address Used for IP Alias
With the IP Alias feature, are there any restrictions on the range of
additional public IP addresses - e.g. do they have to be contiguous, or
within the same subnet, and can they be completely independent of the
primary WAN 1 IP address?
There are no restrictions on the range of additional public IP addresses and they do not need to be contiguous
Join NAT IP Pool option in IP Alias setting
When setting up IP Aliases can you explain what the "Join NAT IP Pool" option does, and in what circumstance it would be used?
Please have a look at the following article. It gives a bit more information on IP alias and the use of NAT Pool:
Email and IP Alias
How to configure the router so that outgoing SMTP mail source address will be seen coming from the IP alias address configured in the router, e.g. public IP Alias of 220.127.116.11
To use the public IP Alias address (18.104.22.168) as the source address, you will need to configure the "Open Port" rule to implement the scenario.