A typical convenient-sized "block" of addresses commonly used is just under 256 addresses. The router almost always is at location xx.yy.zz.1
By default, DHCP will pass out addresses starting at the next address, xx.yy.zz.2 and keep going up toward 254.
By convention, the Address at the end of the range ( xx.yy.zz.255 in this case) is a "magic" address used for broadcast messages, and must not be assigned to an actual device.
To talk to each other easily, the Addresses must "match" in the first three octets of their address, and be different in the last octet, as the above examples do.
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When you produce a reference outside that range, such as asking for Apple's web page at:
http://17.149.160.49
It is sent to your Router, which acts as your "agent" in dealing with the Internet. The Router hides your computer's local address and uses its own Public address (provided automatically by your ISP) so send out packets on your behalf, and listen for the answering packets coming back.
When you use a Router that you control, your computer cannot receive unsolicited queries from the Internet at large because the Router's Firewall discards them. Having a Router that you control means you already have the protection of its Firewall, and need not enable the Firewall built-into your Mac.