📘 03-02: IPv4 Addressing¶

📌 Structure¶

An IPv4 address is 32 bits, written as four decimal octets (0–255):

192.168.1.10
 |   |   | |
 8   8   8 8  bits  = 32 bits total

Each octet is 8 bits, so its range is 0–255 (2^8 = 256 values).

A host needs IP address + subnet mask to know its network, and a default gateway to reach other networks.

🧮 Subnet Mask¶

The subnet mask separates the network portion (1s) from the host portion (0s).

CIDR	Mask	Default class
/8	255.0.0.0	A
/16	255.255.0.0	B
/24	255.255.255.0	C

📌 Private (RFC 1918) Ranges¶

Not routable on the Internet — used inside networks with NAT:

Range	CIDR
10.0.0.0 – 10.255.255.255	/8
172.16.0.0 – 172.31.255.255	/12
192.168.0.0 – 192.168.255.255	/16

📌 Special Addresses¶

Address	Meaning
127.0.0.1	Loopback (localhost) — tests the local stack
169.254.x.x	APIPA — self-assigned when DHCP fails
0.0.0.0	"This/any host" or default route
255.255.255.255	Limited broadcast (local subnet)
224.0.0.0 – 239.255.255.255	Multicast (Class D)

📌 Public vs. Private & NAT¶

Public addresses are globally unique and Internet-routable.
Private addresses are reused internally; NAT translates them to a public address to reach the Internet.

🧭 Scenario: Reading an IP Configuration¶

When troubleshooting a host, do not look at the IP address alone. Read the address, mask, gateway, and DNS settings together.

Symptom	Likely clue	What it suggests
Host has `169.254.x.x`	APIPA address	DHCP did not provide a lease
Host can ping gateway but not websites by name	IP works, DNS fails	Check DNS server and name resolution
Host can ping local devices but not remote networks	No working gateway	Check default gateway and routing
Host IP does not match local subnet	Wrong address or mask	Recheck DHCP scope, static config, or VLAN

Example: a workstation is configured as 192.168.10.75/24 with gateway 192.168.20.1. The host may reach devices in 192.168.10.0/24, but traffic to other networks will fail because the gateway is not in the host's local subnet.

Tip

Key idea — IPv4 = 32 bits, four octets (0–255). The subnet mask marks network vs. host bits. Know the private ranges, loopback (127.0.0.1), and APIPA (169.254.x.x).

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