Mac OS X Leopard - ARCAism

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CHAPTER 9 CONNECTING TO THE INTERNET
Configuring Your Mac for a Broadband
Connection
For your Mac to function on the Internet, it must have the following information:
A qualified IP address: IP addresses are discussed in the “TCP/IP Networking” sidebar;
this is a unique address that identifies your computer on the Internet so that all
information being sent to your computer actually makes it there. You cannot (usually)
just make up an IP address; it must be assigned, or else it will likely not work.
A subnet mask: A subnet mask is used to separate the network address from the host
address. This can be further used within a network to create subnets; breaking up a host
address into subnets allows more effective routing of IP traffic. An example of an IPv4
subnet mask is 255.255.255.0, and any IP address that shares the first three octets in the IP
address is part of the same subnet. If all that sounds foreign to you, don’t worry; just use
the subnet mask your ISP or network administrator gave you.
A gateway address (or router): The router address (also known as a gateway address) is the
IP address of the next upstream router.
A DNS server address: The DNS server is the primary server for your subnet that is
responsible for providing DNS services. A DNS server is responsible for translating a
domain name (that is, apple.com) into an IP address (in other words, 17.254.3.183). You
can list multiple DNS servers if you would like, and if the first one is unable to resolve a
domain name, then the next one listed will be consulted.
Search domains (optional): Search domains are an optional list of domains to search if a
domain address cannot be resolved by any of the DNS servers. This can provide a shortcut
on some networks as well, since it will allow you to address a computer by the host name
alone.
NOTE IPv4 has set aside a number of IP address blocks as private addresses (10.x.x.x,
172.16.x.x, and 192.168.x.x). These private IP addresses are for the creation of private networks
that utilize IP. Many routers (and firewalls) take advantage of these private IP addresses
to perform network address translation (NAT, a.k.a. network masquerading). This allows the
router to be assigned a valid Internet IP address yet assign all the computers behind it private
IP addresses. The router can then act as a gateway between the private network and the Internet,
providing each computer connected to the router with full Internet access without a
dedicated IP address. Although initial implementations of NAT often carried with them some
side effects, most current implementations of NAT provide Internet clients with complete functionality.
Server processes running behind NAT, however, need special considerations and are
limited. This can be used advantageously from a security point of view, and in fact, many firewalls
use NAT combined with port forwarding to hide the actual server from the Internet.
NOTE Port forwarding allows the router or firewall facing the Internet to masquerade as a
server while server requests are actually being passed along to other systems on the private
network. This can be set up so specific services, which use specific ports, can be passed to specific
systems. So, all e-mail traffic using ports 25 (SMTP), 110 (POP3), and 143 (IMAP4) could
point to one server behind the router, while all web traffic using port 80 (HTTP) would be
directed to another.
There are two primary ways to connect your computer to a broadband connection: Ethernet
and Wi-Fi.