Understanding IP Addressing

The Internet Protocol (IP) is the cornerstone of modern networking and is used in most networks, including the biggest of all, the Internet. Here is a quick review of the most important concepts in IP networking:

• IP address assignment: Two strategies exist for IP addressing. The first is manually assigning static addresses, and the second is using a server to assign IP address information dynamically to devices on the network. A common example of dynamic assignment is using a Dynamic Host Configuration Protocol (DHCP) server to provide IP addresses.

• Subnet mask: After a computer is assigned its IP address information, the computer belongs to the subnet defined by the subnet mask and network address. The subnet mask is used to determine which IP addresses are on the same subnet as your computer.

• Router: The router address is where traffic destined to go outside the local subnet is directed. A computer keeps a routing table to determine how to pass packets outside the local subnet. If the routing table does not know where to send a specific packet, it hands it off to the default gateway. Technically, routers and gateways are two different types of devices. However, the terms router and gateway are often used interchangeably to mean a device or interface where packets are sent when the packets are destined for outside the local network. You can see a computer's routing table with the command netstat -nr. The arp -a command will show known hosts in the local subnet.

• DNS server: Domain Name System (DNS) servers provide domain name lookup information that allows you to translate domain names to IP addresses. This lookup ability allows you to reach computers on an IP network through their more humanfriendly host names. Mac OS X allows you to define multiple DNS servers.

Defining a valid IP address, subnet mask, router, and DNS servers allows you to perform other networking services, such as discovering available services on your network. The following figure shows the Network preferences pane and the options associated with configuring an IP address.

IP Addressing Pros and Cons

DHCP is easier for the user to configure because it assigns the IP addresses automatically so you don't have to specify an IP address on each machine. Furthermore, the DHCP server handles all IP address assignment for you, so you don't need to keep track of your IP addresses as they become available or are assigned.

Manual IP addressing, on the other hand, gives a machine a permanent IP address, which is useful if your machine is accessed by its domain name. For example, your system administrator can assign you the domain name imac-17.sales.pretendco.com and map it to your static IP address. In this way, any requests for your domain name are always routed to your computer.

Some TCP/IP networks are set up with the intent that they will never be connected to the Internet and, therefore, do not need specific Internet addresses assigned from IANA (the Internet Assigned Numbers Authority). Examples would be home users, corporations that have secure needs, and training situations. Because of concerns over the growing shortage of IP addresses, a special range of IP addresses called private IP addresses was set aside for these scenarios:

• 10.0.0.010.255.255.255 (also referred to as 10/8)

• 172.16.0.0172.31.255.255 (also referred to as 172.16/12)

• 192.168.0.0192.168.255.255 (also referred to as 192.168/16)

Private IP addresses are sometimes referred to as non-routable IP addresses because the addresses are not routed on the Internet by an Internet service provider (ISP) or Internet routers. The name is misleading because these addresses can be routed by standard routing equipment within private networks and intranets or in conjunction with Network Address Translation (NAT) to connect a private network to the Internet through a valid IP address.