CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [412]
IP version 6, the newest version of the Internet protocol, which will save us all from an Internetless world, uses a 128-bit address instead of IPv4’s 32-bit address. What this means is that there are more possible addresses than with IPv4. A lot more. My favorite illustration is to think of all of the molecules that make up the Earth, and divide them by 7. That’s how many possible IPv6 addresses there are.
The drawback is that IPv6 addresses are not quite as svelte and easy to remember as in IPv4. For example, an IPv6 address looks like this: 2001:0db8:85a3:0000: 0000:8a2e:0370:7334. Not quite as easy to work with as 192.168.1.1, eh?
IPv6 also handles routing and various other things differently than IPv4, but the main things to know are that the IP addresses look remarkably different and there are enough of them to last for a while. There’s no solid plan yet for when everyone is going to switch to IPv6, but it’ll be a big change when it happens.
Table 23-2 Class A, B, and C Addresses
You’ll note that the IP address ranges listed above skip from 126.x.x.x to 128.x.x.x. That’s because the 127 address range (i.e., 127.0.0.1–127.255.255.255) is reserved for network testing (loopback) operations. (We usually just use the address 127.0.0.1 for loopback purposes and call it the localhost address, but any address that starts off with 127 will work just as well.) That’s not the only reserved range, either! Each network class has a specific IP address range reserved for private networks—traffic from these networks doesn’t get routed to the Internet at large. Class A’s private range goes from 10.0.0.1 to 10.255.255.255. Class B has two private address ranges: 172.16.0.1 up to 172.16.255.255 for manually configured addresses and 169.254.0.1 to 169.254.255.254 (link-local addresses) to accommodate the Automatic Private IP Addressing (APIPA) function discussed later. Class C’s private addresses range from 192.168.0.0 to 192.168.255.255.
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NOTE Pinging the loopback is the best way to test whether a NIC is working properly. To test a NIC’s loopback, the other end of the cable must be in a working switch or you must use a loopback device such as a loopback adapter/plug.
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NOTE If APIPA is enabled and the DHCP configured client can’t reach a DHCP server, the client will automatically be configured with an APIPA link-local IP address in the range between 169.254.0.1 to 169.254.255.254 and get a Class B subnet mask of 255.255.0.0 until the DHCP server can be reached.
Subnet Mask The subnet mask is a value that distinguishes which part of the IP address is the network address and which part of the address is the host address. The subnet mask blocks out (or masks) the network portions (octets) of an IP address. Certain subnet masks are applied by default. The default subnet mask for Class A addresses is 255.0.0.0; for Class B, it’s 255.255.0.0; and for Class C, 255.255.255.0. For example, in the Class B IP address 131.190.4.121 with a subnet mask of 255.255.0.0, the first two octets (131.190) make up the network address, and the last two (4.121) make up the host address.
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EXAM TIP The CompTIA A+ certification exams do not require you to break down IP addresses and subnet masks into their binary equivalents or to deal with non-standard subnet masks such as 255.255.240.0, but you should know what IP addresses and subnet masks are and how to configure your PC to connect to a TCP/IP network.
A New Kind of Port
The term “port” has several meanings in the computer world. Commonly, port defines the connector socket on an Ethernet NIC, where you insert an RJ-45 jack. That’s how I’ve used the term for the most part in this book. It’s now time to see another use of the word ports.
In TCP/IP, ports are 16-bit numbers between 0 and 65,535, assigned to a particular