Why is that? Level3 is one of my upstreams and I've had two /23s from them for a few years. I'm pretty sure they still have some addresses available, although they might be harder to get nowadays.
You can get provider-aggretable (PA) IP addresses from your ISP, but they can only be used with that ISP. You can also get provider-independent (PI) addresses, which you can use and move between ISPs with BGP. If they're using Zayo and Level 3 at the same time, you would need PI-IPs that you can move to the other ISP when one ISP is down.
Or, another option is that they're doing CGNAT on all of their client traffic.
> You can get provider-aggretable (PA) IP addresses from your ISP, but they can only be used with that ISP.
Sorry, this is incorrect -- I'm a network engineer and I've been doing exactly this for years. I advertise "Level3 IPs" to other ISPs/peers, I advertise "non-Level3 IPs" to Level3, hell, I even "re-advertise" some of my customers' PI addresses (that they advertise to me) up to Level3.
Actually, I can advertise any prefixes I want via Level3 -- even yours (assuming you had your own).
n.b.: Now, some ISPs may require an LOA (or similar) before they'll accept certain prefixes from you but that's a (easily solved) procedural issue -- not a technical issue.
Sure, there's no technical issue since a prefix is a prefix is a prefix. It's all just bits. If you can convince your upstream/peers to accept a route, there's nobody to stop you. I haven't run an ISP, so maybe I haven't had to push hard enough, but my transit providers' willingness to accept someone else's PA block has been nil.
