Maximum IP Header Size (IPv4): Understanding the 60-Byte Limit
For IPv4, the header length is encoded in the header’s IHL (Internet Header Length) field, which is 4 bits wide, meaning it can represent values from 0 to 15. Because IHL counts 32-bit words (not bytes), the maximum IPv4 header size is:
So, the correct choice is (iv) 60 bytes.
Key idea: IPv4 headers can include options, and those options increase the header size up to the IHL-imposed maximum.
Keywords: IPv4 IHL Options Header length
IPv4 Header Length (IHL) and Options Explained
Compute the maximum IPv4 header size from IHL
- 1Step 1
In IPv4, the header length (IHL) is a 4-bit value, so the maximum numeric value is 15.
- 2Step 2
IHL is measured in 32-bit words. Since 1 word = 4 bytes, convert words to bytes.
- 3Step 3
Maximum header bytes = bytes.
- 4Step 4
The result corresponds to option (iv) 60 bytes.
Why options matter
IPv4’s base header is 20 bytes, but the options area can extend the header. The IHL field is what caps how large that extension can get (max 60 bytes).
Don’t confuse IP header size with full packet size
The maximum IPv4 header size (60 bytes) does not mean the entire IP packet is only 60 bytes—payload and (upper-layer) data add much more.
Quick answer mapping
| Question option | Header size claimed | Verdict |
|---|---|---|
| (i) 32 bytes | bytes | Not maximum |
| (ii) 64 bytes | bytes | Too large for IPv4 IHL limit |
| (iii) 30 bytes | bytes | Not consistent with 32-bit word granularity |
| (iv) 60 bytes | bytes | ✅ Correct (IPv4 maximum) |
Keywords: Payload MTU Fragmentation Header options
IPv4 Header Length Logic (From Field to Bytes)
IHL is 4 bits
Field constraintIHL can encode values up to 15."
IHL is in 32-bit words
Unit conversionEach unit equals 4 bytes."
Max header size computed
Maximum resultMax header = bytes."
Common confusions
Knowledge Check
What is the maximum header size of an IPv4 packet?