Many Internet Service Providers implement a network configuration technique known as carrier-grade NAT or large-scale NAT. Wikipedia describes carrier-grade NAT:
“Carrier-grade NAT (CGN), also known as large-scale NAT (LSN), is an approach to IPv4 network design in which end sites, in particular residential networks, are configured with private network addresses that are translated to public IPv4 addresses by middlebox network address translator devices embedded in the network..."
Carrier-grade NAT, or CGN from here on, solves an IP addressing problem for ISPs, but causes a variety of problems for their customers. These problems are caused by CGN essentially interrupting what should be end-to-end connections, such as those established by Screens when connecting to a remote computer. For further information, see Assessing the Impact of NAT444 on Network Applications.
You can check whether you’re on a CGN network by visiting a Web site such as ipaddress.com or whatsmyip.com and comparing the IP address listed to the public/WAN IP address assigned to your Internet gateway. To find your gateway’s public IP address, check the status page in the administrative interface. If the two IP addresses differ, you’re likely on a CGN network. If so, you will have trouble establishing Screens connections to the network in question, but it may be possible to request a static IP address from your ISP to avoid connectivity issues.