I have been working on setting up my old Ubuntu laptop to use as an SSH server. I can access it over a local network, but am unable to access it over the internet. I am using Google's SSH Client software in both cases. I have ensured that the ports do match. When I try, I get this message:

ssh: connect to host 2601:8d:8380:21:6424:49ec:fef1:4bcc port 2213: Connection refused
NaCl plugin exited with status code 255.

This is before it even prompts me for a password. I do not know much about networking, so if you need me to give any more information, please show me how (give me the command line commands, for example).

Thanks.

you need to port forward port 22 ( standard ssh port ) to your local macine you also need a static ip adress both on the lan and facing out to the internet.

I have the port set to a different value that matches on both the server and the client (as various articles recommended I do that). However, I have tried port 22 as well, and I get the same error. I believe the ip address is static, as it has remained the same each time I've checked it. Is there a way for me to be sure?

a number of sites will report your outward facing ip try:
https://www.grc.com/x/ne.dll?bh0bkyd2

you can also use this site to check that your ssh port is responding amongst other things

I took a look at that site, and it gave me something like this: c-..-...-...-....hsd1.nj.comcast.net (I've replaced the numbers with dots). It also gave me an IPv4 address. However, what I've been using, and what has worked over my local network, is the IPv6 address. It is also the only one that *seems* to be connecting to my server computer at all. The others report nothing, the IPv6 address reports a refused connection.

Din't know much about ipv6 but if you don't have an open connection to your ssh port your firewall must be blocking it, presumably the one in your router as you can connect locally, so thats where you have to forward your port to your local machine running the ssh server.

So I would need to handle the firewall in the router, not on my computer. How would I go about that?

That will depend on your router, most have some sort of web interface where you can set various options, check your router manual or the website of the supplier of the router, can't really help with that uunless you happen to use a BT homehub 3 router.

I am using Comcast, and have been able to get into the settings.

I have tried port forwarding, port triggering, and DMZ, and all of them have failed. I have tried using both the IPv4 and IPv6 addresses for the Ubuntu device, but all reports I run online tell me that the port is closed. It seems the problem is with the Comcast router (or with wrong input on my part).

seems to be problem with the router, I would suggest asking for help to set it up on the network forum you basicaly cant do anything until you get the port forwarding working, as i said I know nothing about comcast so cant help with that, sorry.