1. It's part of the Internet addressing architecture, the fact that private networks were designed for home and offices (LANs), and in this case, your modem has the Public IP address assigned by your ISP. Such device assigns you a Private IP address via DHCP, and it does NAT all your requests.
By the way, ifconfig command shows the IP address that was assigned either statically or dynamically(via DHCP) to your NIC. If you need to know your Public IP address you can search for an IP Address lookup service in internet.
2. The subnet masks and the IP addresses range are determined by your modem configuration. There are some modems that you can configure to set your own values.