Cisco CCNA Preparation · lesson 5 of 36 · 25 min
IPv4 Address Structure
An IPv4 address is a 32-bit number written as four octets in dotted-decimal notation (e.g. 192.168.1.10). Each octet is 8 bits (0–255). Every IP address has two parts: the network portion (identifies the network) and the host portion (identifies the device on that network). The subnet mask defines where the split is.
Private address ranges (RFC 1918)
10.0.0.0/8 · 172.16.0.0/12 · 192.168.0.0/16 — not routable on the public internet; used inside LANs.
Subnet Mask and CIDR
A subnet mask uses 1s to mark the network bits and 0s for host bits. CIDR notation (Classless Inter-Domain Routing) expresses this as a prefix length — the number of 1 bits. /24 = 255.255.255.0 = 24 network bits, 8 host bits.
Usable hosts formula
Usable hosts = 2ⁿ − 2 where n = number of host bits. Subtract 2 for the network address (all host bits 0) and broadcast address (all host bits 1).
Subnetting Worked Example
Problem: You have the network 192.168.10.0/24 and need 4 subnets. What prefix do you use?
Step 1: You need at least 4 subnets → borrow 2 bits (2² = 4). New prefix = /24 + 2 = /26.
Step 2: Host bits remaining = 32 − 26 = 6. Usable hosts = 2⁶ − 2 = 62 per subnet.
Step 3: Block size = 256 − 192 = 64. Subnets:
Special Addresses
Flashcard Drill
/24 subnet mask 255.255.255.0 — 8 host bits — 254 usable hosts. /26 subnet mask 255.255.255.192 — 6 host bits — 62 usable hosts. /30 subnet mask 255.255.255.252 — 2 host bits — 2 usable hosts (point-to-point links). Usable hosts formula 2ⁿ − 2 where n = host bits. Subtract 2 for network and broadcast. 127.0.0.1 Loopback address — tests local TCP/IP stack. APIPA 169.254.x.x — auto-assigned when DHCP fails. RFC 1918 private ranges 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — not routable on the internet. CIDR Classless Inter-Domain Routing — expresses subnet mask as prefix length (/24, /26, etc.). Network address First address in a subnet — all host bits are 0. Cannot be assigned to a host. Broadcast address Last address in a subnet — all host bits are 1. Cannot be assigned to a host.
Knowledge Check
IPv4 & Subnetting — 10 Questions
How many usable host addresses does a /26 subnet provide?
- 64
- 62
- 30
- 126
Answer: 62
/26 has 6 host bits. 2⁶ − 2 = 62 usable hosts.
What is the subnet mask for /27?
- 255.255.255.192
- 255.255.255.224
- 255.255.255.240
- 255.255.255.248
Answer: 255.255.255.224
/27 = 27 network bits. Last octet: 11100000 = 224. Mask = 255.255.255.224.
192.168.1.0 is a valid host address in the 192.168.1.0/24 subnet.
- True
- False
Answer: False
192.168.1.0 is the network address (all host bits = 0) — it cannot be assigned to a host.
The loopback address used to test the local TCP/IP stack is _____.
Answer: 127.0.0.1
127.0.0.1 is the loopback address — traffic never leaves the device.
Which address range is assigned automatically when DHCP fails?
- 10.0.0.0/8
- 172.16.0.0/12
- 169.254.0.0/16
- 192.168.0.0/16
Answer: 169.254.0.0/16
APIPA (Automatic Private IP Addressing) assigns 169.254.x.x when DHCP is unavailable.
You need a subnet for a point-to-point WAN link (2 hosts only). Which prefix is most efficient?
- /28
- /29
- /30
- /31
Answer: /30
/30 provides exactly 2 usable host addresses — perfect for point-to-point links.
Which class of IPv4 address has a default mask of /8?
- Class A
- Class B
- Class C
- Class D
Answer: Class A
Class A (1–126) has a default /8 mask (255.0.0.0).
The network 192.168.5.0/24 is divided into /26 subnets. How many subnets are created?
- 2
- 4
- 6
- 8
Answer: 4
Borrowing 2 bits from /24 to get /26 creates 2² = 4 subnets.
10.0.0.0/8 is a private address range not routable on the public internet.
- True
- False
Answer: True
10.0.0.0/8 is one of three RFC 1918 private ranges — not routable on the internet.
The formula for usable hosts in a subnet is 2ⁿ − _____ where n is the number of host bits.
Answer: 2
Subtract 2 for the network address and broadcast address.
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