Question bank — IPv4 — address format, classes, subnetting, CIDR notation

False. More network bits leaves fewer host bits, so the network is smaller. A /30 holds 2 usable hosts; a /8 holds millions.

In classic counting yes, but RFC 3021 allows /31 on point-to-point links where both addresses are usable — there's no need for a broadcast when only two ends exist.

True by the leading-bit rule, but under CIDR the class is ignored — routing uses the prefix you write, not the historic class.

True. Twenty-four consecutive ones is 11111111.11111111.11111111.00000000, which reads as 255.255.255.0. They are two spellings of one boundary.

False. With host bits all ones it is the broadcast address of 192.168.1.0/24, reserved to reach every host — never assigned to a single machine.

False. 10.0.0.0/8 is private (RFC 1918); routers drop it publicly. It must be translated to a public address first.

False. 169.254.0.0/16 is link-local (APIPA), self-assigned precisely because DHCP failed — it only reaches the local link.

False. They aggregate only if they are adjacent and aligned — the /23 boundary requires the first /24's third octet to be even (e.g. .0 and .1, not .1 and .2).

False. All of 127.0.0.0/8 is loopback — traffic never leaves the host. Only the local machine sees it.

Error: forgot the minus 2. Six host bits give 64 patterns, but network (all-zeros) and broadcast (all-ones) are reserved ⇒ 62 usable.

Error: 1 bit gives only $2^1=2$ subnets. You need $2^s\ge 4\Rightarrow s=2$ bits. Each borrowed bit doubles the subnet count.

Error: for /26 the block size is 64, so network addresses must be multiples of 64 in the last octet: .0, .64, .128, .192. 40 isn't a boundary; the network is .0.

Error: that's classful thinking. CIDR is classless — you can carve a /24 into /30s (2 hosts each) or /29s (6 hosts each) freely.

Error: it is the limited broadcast address, reserved to reach everything on the local segment. It cannot name a network or a host.

Error: $10100000_2=160\ne 168$. Conversion must be exact: $168=128+32+8=10101000$. See Binary and Positional Number Systems.

Error: the private range is 172.16.0.0/12, meaning 172.16–172.31 only. 172.15.x.x is public — "near" doesn't count, the prefix boundary does.

Error: /26 has 26 network bits (6 host bits, 62 hosts); /24 has 8 host bits (254 hosts). Bigger prefix = smaller address pool.

Hosts lose the all-zeros (network) and all-ones (broadcast) patterns — those name the network, not machines. Subnets have no such reserved patterns; all $2^s$ combinations are legitimate subnets.

Adjacent networks sharing a common prefix collapse into one route (aggregation/supernetting). One /23 entry replaces two /24 entries, so routers store far fewer lines.

So routers store routes to networks (thousands), not to every device (billions). They match only the network part — smaller tables, faster lookups.

Sizes were fixed with huge gaps: Class C = 254 hosts, Class B = 65k, nothing between. An org needing 2000 hosts grabbed a whole Class B, stranding ~63k addresses.

$2^{32-n}$ is the total host count; the block size is that count expressed within the single octet the mask breaks. When many host bits spill past one octet you measure spacing in the breaking octet only.

$2^{32}$ ran out under global demand even with subnetting and NAT. IPv6 uses 128 bits ($2^{128}$), effectively unlimited.

Longest-prefix match: the most specific (longest) matching prefix wins, because it describes the destination more precisely than a broader route.

A /32 has zero host bits, so the single address is both its own network and broadcast — it names exactly one host (used for host routes).

$2^2-2=2$ usable hosts — exactly right for a point-to-point link between two routers, one address per end.

Yes — with zero network bits it matches every address. It is the default route ("send anything I don't otherwise know how to reach here").

Host bits $32-n\to 0$, so usable hosts $2^0-2=-1$ formally, i.e. the "minus 2" formula breaks down. /31 and /32 are the special cases handled by their own rules.

By leading bit 0 it looks Class A, but 127.0.0.0/8 is carved out entirely for loopback — a reserved exception, not usable as a normal Class A network.

No — they differ by 1. For /26: .63 is broadcast of the .0 block; .64 is the next network. Consecutive but never equal.