CompTIA Network+ Preparation · lesson 7 of 30 · 25 min
IPv4 Addressing Review
An IPv4 address is 32 bits written as four octets in dotted-decimal notation: 192.168.1.100. Each octet is 8 bits (0–255).
Address classes (legacy, still tested):
Class A: 1.0.0.0 – 126.255.255.255. Default mask /8. Large networks (16 million hosts).
Class B: 128.0.0.0 – 191.255.255.255. Default mask /16. Medium networks (65,534 hosts).
Class C: 192.0.0.0 – 223.255.255.255. Default mask /24. Small networks (254 hosts).
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.
Loopback: 127.0.0.1 (localhost). APIPA: 169.254.0.0/16 (auto-assigned when DHCP fails).
CIDR and Subnet Masks
CIDR (Classless Inter-Domain Routing) notation uses a slash followed by the number of network bits: 192.168.1.0/24. This replaced classful addressing and allows flexible subnet sizes.
Subnet mask — 32-bit value where 1s represent the network portion and 0s represent the host portion. /24 = 255.255.255.0.
Key calculations for any subnet:
Number of hosts = 2^(host bits) – 2. Subtract 2 for network address and broadcast address.
Network address — all host bits set to 0. First address in the subnet.
Broadcast address — all host bits set to 1. Last address in the subnet.
Usable host range — network address + 1 to broadcast address – 1.
Example — 192.168.1.0/26:
Host bits = 32 – 26 = 6. Usable hosts = 2^6 – 2 = 62. Subnet mask = 255.255.255.192.
Network address: 192.168.1.0. Broadcast: 192.168.1.63. Usable range: 192.168.1.1 – 192.168.1.62.
Subnetting a /24 into /26s: 2^2 = 4 subnets. Each has 62 usable hosts. Subnets: .0/26, .64/26, .128/26, .192/26.
Quick Method
Block size = 256 – subnet mask last octet. For /26: 256 – 192 = 64. Subnets start at 0, 64, 128, 192. The broadcast of each subnet is one less than the next subnet’s start.
Key Terms
Key Terms
Knowledge Check — 10 Questions
Network+ — IPv4 Subnetting
How many usable hosts does a /28 subnet provide?
- 30
- 16
- 14
- 12
Answer: 14
/28 = 4 host bits. 2^4 = 16 addresses. 16 – 2 = 14 usable hosts (subtract network and broadcast).
What is the subnet mask for /26?
- 255.255.255.128
- 255.255.255.192
- 255.255.255.224
- 255.255.255.240
Answer: 255.255.255.192
/26 = 26 network bits. Last octet: 11000000 = 128+64 = 192. Mask = 255.255.255.192.
For 192.168.10.0/27, what is the broadcast address?
- 192.168.10.30
- 192.168.10.31
- 192.168.10.32
- 192.168.10.63
Answer: 192.168.10.31
/27 = 5 host bits. Block size = 32. Subnet 0: network=.0, broadcast=.31, hosts=.1–.30.
Which address range is an RFC 1918 private range?
- 172.32.0.0/12
- 192.168.0.0/16
- 10.0.0.0/16
- 169.254.0.0/16
Answer: 192.168.0.0/16
RFC 1918 private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Note: 172.16–172.31 (not 172.32). 169.254 is APIPA.
APIPA addresses (169.254.x.x) indicate:
- A successful DHCP assignment
- DHCP failed — the device assigned itself an address
- A static IP configuration
- A public IP address
- Answer: DHCP failed — the device assigned itself an address
APIPA (169.254.0.0/16) is auto-assigned when a device cannot reach a DHCP server. It only allows local subnet communication.
How many /26 subnets can be created from a /24 network?
- 2
- 4
- 8
- 16
Answer: 4
Borrowing 2 bits from a /24 gives /26. 2^2 = 4 subnets. Each has 62 usable hosts.
The loopback address 127.0.0.1 is used for:
- Default gateway
- Testing the local TCP/IP stack without sending traffic on the network
- DHCP server
- DNS server
- Answer: Testing the local TCP/IP stack without sending traffic on the network
127.0.0.1 (localhost) loops back to the local machine — used to test that TCP/IP is working without network hardware. A /30 subnet is commonly used for:
- Large office networks
- Point-to-point WAN links — only 2 usable hosts needed
- Server farms
- Wireless networks
- Answer: Point-to-point WAN links — only 2 usable hosts needed
/30 = 2 usable hosts — perfect for point-to-point links between two routers where only 2 IPs are needed.
What is the network address of 10.0.0.130/25?
- 10.0.0.0
- 10.0.0.128
- 10.0.0.129
- 10.0.0.255
Answer: 10.0.0.128
/25 block size = 128. Subnets: 10.0.0.0 (hosts .1–.126) and 10.0.0.128 (hosts .129–.254). .130 falls in the second subnet.
The formula for usable hosts in a subnet is 2^n – _____, where n is the number of host bits.
Answer: 2
Subtract 2: one for the network address (all host bits 0) and one for the broadcast address (all host bits 1).
Try it: IPv4 subnetting drill
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