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Here is the link of website where they say we can have numbers greater than 127 in 1st octet https://www.informit.com/articles/article.aspx?p=433329&seqNum=5#:~:text=10-,128%E2%80%93191,-C

Sameera Khatoon
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  • Octets are simply to make it easier for humans to read, and they have no meaning to IPv4. An IPv4 address is just a 32-bit unsigned integer (a 128-bit unsigned integer for IPv6). See this two part answer that explains IPv4 math. – Ron Maupin Aug 07 '22 at 16:11

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"Octet" comes from an group of eight bits, often also called (unsigned) "byte". An octet can have one of 2^8=256 positive values, 0 through 255. An IPv4 address is actually 32 bit, usually represented as four dotted octets for human readability.

Not all values can be used in the first octet of a host address, as several are reserved for special purposes. Most significantly,

  • 0 is reserved for the "current network"
  • 127 is reserved for the local host's loopback address (most often only 127.0.0.1 is used)
  • 224-239 are reserved for multicast (as destination address only)
  • 240-255 are reserved for "future use" (which isn't going to happen); 255.255.255.255 is used as the limited broadcast destination address, ie. all hosts in this network

As a side note: the linked page talks all about network classes which are long obsolete and may be interesting for historical reasons only. Network classes were obsoleted decades ago by CIDR (RFCs 1518 & 1519).

Zac67
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