Two ways of writing an address. Are they the same host, or two different ones?
An address is 32 bits, but it can be written down in more than
one way. A part that starts with 0 was read as octal, a part that
starts with 0x as hexadecimal, and a short form like
192.168.257 reads the last part as a whole number rather than as an
octet. Historic parsers accepted all of it. Modern ones mostly do not — and the
difference between the two is a well-known way past a filter that only checks the
text.
Are these the same host?
First
10.0.0.1
Second
10.0.0.255
Pick the one you think is right.
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How they read
10.0.0.1 = 10.0.0.1
dotted decimal
10.0.0.255 = 10.0.0.255
dotted decimal
They are different addresses. Read 10.0.0.1 as 10.0.0.1, and 10.0.0.255 as 10.0.0.255 — one bit or one octet apart. The normal way to write an address.
00001010 00000000 00000000 00000001
00001010 00000000 00000000 11111111
An address is four octets, and each octet is eight bits. The spellings differ in how the octets are written, not in what they mean:
192.168.1.1 — four decimal octets. 192 is
11000000.192.168.001.001 — the same, with the leading zeros. Leading
zeroes do not make an octet octal on their own; they are just padding.0300.0250.0001.0001 — octal. 0300 is 192 in
decimal, and 0250 is 168.0xc0.0xa8.0x01.0x01 — hexadecimal. 0xc0 is 192 and
0xa8 is 168.192.168.257 — the short form. The last part is not an octet at
all: it is the low 16 bits of the address, so 257 = 0x0101, which splits into
the last two octets 1.1.That is the whole lesson, and it has a practical edge: a system that accepts
0177.0.0.1 as 127.0.0.1 is a system whose "is this
address private?" check can be written in a way that only matches the dotted
decimal form. The address is the same host either way — the filter is what
differs.
Modern parsers reject most of this on purpose. A hostname or an address that comes from a user should be read one way, and the way that is written down, not in whatever base it looks like it might be in.
This is the daily round, the same for everyone on
2026-10-23. /notation.htm?p=1, ?p=2,
?p=3 … walk through the pairs one at a time.
Like this one? Subnet Sweeper is the same bits with a mask in it, Public or Reserved? asks what an address space is for, and the network tools will resolve a name back to an address.