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Network Access, Internet, and Transport Layer Protocols and Concepts

TCP/IP is a protocol suite, including (but not limited to): IPv4 and IPv6 at the Internet layer; TCP and UDP at the Transport layer; and a multitude of higher-level protocols, including Telnet, FTP, SSH, and many others. Let us focus on the lower layer protocols, spanning from the Network Access to Transport layers. Some protocols, such as IP, fit neatly into one layer (Internet). Others, such as Address Resolution Protocol (ARP), help connect one layer to another (Network Access to Internet in ARP’s case).

MAC Addresses

A Media Access Control (MAC) address is the unique hardware address of an Ethernet network interface card (NIC), typically “burned in” at the factory. MAC addresses may be changed in software.

Note
Burned-in MAC addresses should be unique. There are real-world exceptions to this, often due to mistakes by NIC manufacturers, but hardware MAC addresses are considered unique on the exam.

Historically, MAC addresses were 48 bits long. They have two halves: the first 24 bits form the Organizationally Unique Identifier (OUI) and the last 24 bits form a serial number (formally called an extension identifier).

Organizations that manufacture NICs, such as Cisco, Juniper, HP, IBM, and many others, purchase 24-bit OUIs from the Institute of Electrical and Electronics Engineers (IEEE), Incorporated Registration Authority. A list of registered OUIs is available at IEEE Standards.

Juniper owns OUI 00-05-85, for example. Any NIC with a MAC address that begins with 00:05:85 is a Juniper NIC. Juniper can then assign MAC addresses based on their OUI: the first would have been MAC address 00:05:85:00:00:00, the second 00:05:85:00:00:01, the third 00:05:85:00:00:02, etc. This process continues until the serial numbers for that OUI have been exhausted. Then a new OUI is needed.

EUI-64 MAC Addresses

The IEEE created the EUI-64 (Extended Unique Identifier) standard for 64-bit MAC addresses. The OUI is still 24 bits, but the serial number is 40 bits. This allows for far more MAC addresses, compared with 48-bit addresses. IPv6 auto-configuration is compatible with both types of MAC addresses.

IPv4

IPv4 is Internet Protocol version 4, commonly called “IP.” It is the fundamental protocol of the Internet, designed in the 1970s to support packet-switched networking for the United States Defense Advanced Research Projects Agency (DARPA). IPv4 was used for the ARPAnet, which later became the Internet.

IP is a simple protocol, designed to carry data across networks. It is so simple that it requires a “helper protocol” called ICMP (see below). IP is connectionless and unreliable: it provides “best effort” delivery of packets. If connections and reliability are required, they must be provided by a higher-level protocol carried by IP, such as TCP.

IPv4 uses 32-bit source and destination addresses, usually shown in “dotted quad” format, such as “192.168.2.4.” A 32-bit address field allows 2^32, or nearly 4.3 billion, addresses. A lack of IPv4 addresses in a world where humans (and their devices) outnumber available IPv4 addresses is a fundamental problem: this was one of the factors leading to the creation of IPv6, which uses much larger 128-bit addresses.