Physical and Logical Addressing

Why networks use addresses

Networks need identifiers so data can be associated with the right interfaces and endpoints. Two important forms are MAC addresses, used as local-link identifiers, and IP addresses, used as logical network addresses.

They are related but not interchangeable. They have different formats and responsibilities.

MAC addresses

An Ethernet MAC address is 48 bits and is commonly written as six hexadecimal pairs, such as 02:1A:2B:3C:4D:5E. Hexadecimal uses digits 0–9 and letters A–F.

A MAC address identifies an interface for local-link purposes. Later lessons explain how Ethernet uses it.

U/L and I/G bits

The two least-significant bits of the first octet have introductory meaning:

• The I/G bit is the least-significant bit. 0 means an individual address; 1 means a group address.
• The U/L bit is the next bit. 0 indicates universally administered; 1 indicates locally administered.

For 02:1A:2B:3C:4D:5E, the I/G bit is 0 and the U/L bit is 1, so it is an individual, locally administered MAC address.

IP addresses

An IP address is a logical network address. IPv4 is commonly written in dotted decimal, such as 192.0.2.10. IPv6 uses hexadecimal groups separated by colons.

MAC addresses support local-link identity; IP addresses provide logical network addressing across connected networks. Detailed IPv4, IPv6, and subnet concepts come in the Network Layer module.

Inspect address formats

Choose each sample to identify its format and, for MAC examples, the introductory U/L and I/G meaning.

Inspect an address format

MAC and IP addresses have different jobs and recognizable written formats.

02:1A:2B:3C:4D:5E01:00:5E:00:00:FB192.0.2.10

Knowledge check

Knowledge check: What kind of address is 02:1A:2B:3C:4D:5E?

A MAC address written in hexadecimalAn IPv4 address in dotted decimalA web address

Check answer

Knowledge check: What do I/G = 1 and U/L = 1 indicate?

A group address and a locally administered addressAn individual and universally administered addressAn IPv4 broadcast address

Check answer

Knowledge check: Which comparison is correct?

MAC supports local-link identity; IP provides logical network addressingMAC and IP are the same formatIP is only a physical cable label

Check answer