Subnetting an ISP Block: VLSM Allocation for 80.70.56.0/21

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Nov 3, 2026

We’re given an ISP block of $80.70.56.0/21$ . Under IPv4 CIDR, a $/21$ prefix means the network portion is 21 bits and therefore the block contains:

total addresses $=2^{32-21}=2^{11}=2048$
block size for a $/21$ is $2^{(32-21)}=2048$ addresses
The valid address range runs from the block’s network address up to its broadcast address.

keywordCIDR[/keyword] keywordnetwork address[/keyword] keywordbroadcast address[/keyword] keywordaddress block[/keyword]

Because the question only asks (a) the number and range of addresses in the ISP block, we focus on the $/21$ boundaries.

Find the number and range of addresses in 80.70.56.0/21

1
Step 1
A /21 means $32-21=11$ host bits. Total addresses in the block are $2^{11}=2048$ .
2
Step 2
For $/21$ , the third octet boundary step is $256- (remainder)$ ; equivalently, a /21 spans $256/2=128$ values in the 3rd octet because $21-16=5$ bits cover the 3rd octet and leave $3$ bits in the 3rd octet to vary (so the step is $2^7=128$ addresses per block in 3rd-octet terms). Starting at third-octet value 56, the block runs from 56 to 63 inclusive (since $56 + 8 = 64$ ).
3
Step 3
Network address is 80.70.56.0. The last octet changes within that span, so the broadcast address is 80.70.63.255.
4
Step 4
Address range in the ISP block is from 80.70.56.0 through 80.70.63.255, inclusive.

Rule of thumb for /21 ranges

For IPv4, a $/21$ covers 8 values in the 3rd octet (because $2^{21-16}=2^5=32$ ? the practical result is: $/21$ groups third-octet blocks in chunks of 8 when starting from any valid boundary). For 80.70.56.0/21 specifically, you can directly verify it spans 80.70.56.0–80.70.63.255.

(a) Number and range of addresses in the ISP block

Number of addresses: $2048$
Range of addresses: $80.70.56.0 \text{ to } 80.70.63.255$

keywordtotal addresses[/keyword] keywordsubnet start[/keyword] keywordsubnet end[/keyword]

ISP Block Address Capacity (/21)

Capacity of 80.70.56.0/21

Address-count and range sanity checks

Knowledge Check

Question 1 of 4

Q1Single choice

For an IPv4 CIDR block /21, how many total addresses are in the block?

1024

2048

4096

512

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