Fun-Day Friday – Book Review – Networking for Systems Administrators chapter 3

This chapter is all about “Layer 3” (or the “Network Layer.”) Where Ethernet requires a MAC address to know where to send frames, Layer 3 in TCP/IP systems is the “IP address” layer. Addressing is either going to be IP version 4, or IP version 6. This is either a 32bit or 128bit number. Lucas mentions the Dynamic Host Configuration Protocol (DHCP) often used by workstations, versus statically setting an IP as you would do on a system that is designed to be a server.

He explains the notations used to represent the addresses. The most common notation for IPv4 is the dotted quad, which is four decimal numbers from 0 to 255, separated by a “dot.” He goes into how applying a “subnet mask” allows us to cut a network into smaller “subnet” networks. All addresses on the same subnet are able to talk to each other directly. Any address that wants to talk to an address outside of its subnet MUST go through a router first.

He covers the Classless Inter-Domain Routing notation, which applies the number of bits in the subnet mask to the end of the address to represent the network, rather than giving the IP and subnet addresses separately. There’s a handy chart for this on page 47.

There is a brief discussion about multi-homing, the loopback, and private networks with network address translations (NAT.) He covers a few tools to inspect, configure, and troubleshoot IP, including traceroute, ping, ifconfig, and netstat.

Next, Lucas introduces IPv6, which are often depicted as a colon separated list of alpha-numeric characters. The address is up to eight sets of four hexadecimal numbers. This represents 16 bits per set between the colons. Because this is cumbersome, shorthand notation may be used. Leading zeros from a hex section may be removed from the notation. This means a section of all zeros may be empty, so you get two colons back to back. Multiple sets of zeros may be “squeezed out” to just a single “double colon” set. Because of this special shorthand notation, only do a double colon ONCE per IP address.

IPv6 has an “autoconfiguration” facility, which allows clients to learn their IP address, as well as the router’s IP address. This facility works for a /64 network, and the protocol is “router discovery.” It doesn’t allow for assigning DNS servers, so DHCPv6 may still be necessary.

The “localhost” address in IPv6 has a special notation. The “::1” shorthand represents this loopback address.

Another special case in IPv6 is the “link-local” address. This is an auto-configured network address that begins with “fe8.” Each interface gets its own link-local, and they can be the same IP, so the OS attaches the interface name to the address. All IPv6 hosts on the same ethernet network can find each other through link-local communications. A link-local address usually appears with a “%” (percent sign) and the interface name or number at the end of the address.

The rest of the chapter includes the same troubleshooting, inspection, and configuration tools as the IPv4 sections, as well as some discussion on IPv6 tunneling (usually used to test IPv6 when your ISP doesn’t offer it yet,) as well as some discussion on how some operating systems decide which addressing to use by default, and when, where, and why you might choose to use IPv4 or IPv6 in your environment.

The next chapter will cover the TCP/IP “transport” layer (Layer 4.)

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