A host is configured with the address 172.20.35.77/20. What is the network address of its subnet?
LPIC-1 Exam 102-500, objective 109. Networking fundamentals hard
Machine-checked — no person has signed for it. This question was read against the source cited below by an automated pass, which found no contradiction. That is a weaker claim than it sounds: the same kind of process wrote the question, so it can confirm its own mistake.
Treat it as a good draft rather than as settled fact, and read the source below before you rely on it. It is not used in mock exams here — only questions a person has signed for are.
How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Not correct 172.20.0.0
Wrong. 172.20.0.0 would be the network address for a /16 prefix, where the whole third octet is host space. A /20 splits the third octet.
Correct 172.20.32.0
Correct. A /20 is the mask 255.255.240.0. ANDing the third octet, 35 & 240 = 32, and the fourth octet becomes 0, giving 172.20.32.0.
Not correct 172.20.35.0
Wrong. 172.20.35.0 assumes a /24, where only the last octet is host space. With a /20 the boundary falls inside the third octet.
Not correct 172.20.16.0
Wrong. 172.20.16.0 is the previous /20 block; it covers 172.20.16.0 through 172.20.31.255, and 35 falls above that range.
Why
A /20 mask is 255.255.240.0, so /20 blocks step by 16 in the third octet: 0, 16, 32, 48 and so on. 35 falls in the block that starts at 32, so the network address is 172.20.32.0 and the broadcast address is 172.20.47.255. Bitwise AND of the address with the mask always yields the network address.
Where this comes from
- Cited
- LPI exam objective 109.1
- What it says
- Calculate the network address of a host given its address and subnet mask.
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 109 and space the ones you get wrong.
More questions on this objective
- A subnet is defined as 192.168.10.0/26. How many addresses in it can actually be assigned to hosts? machine-checked
- In IPv6, which address is the loopback address, equivalent in role to 127.0.0.1 in IPv4? machine-checked
- Which is the correct fully compressed form of the IPv6 address 2001:0db8:0000:0000:0000:ff00:0042:8329? machine-checked
- Which pairing of a service with its default port number is correct? machine-checked
- DNS normally uses UDP port 53. In which situation does a DNS client or server fall back to TCP on port 53? machine-checked
- Which file on a Linux system maps service names such as ssh, smtp and http to their port numbers and transport protocols? machine-checked