2

let's say that we have a router with address: 172.31.219.33/27 from this I can tell several things:

  1. Subnet mask: 255.255.255.224
  2. Broadcast address (here I am not sure if last digit should 64 or 65 - see later): 172.31.219.64
  3. Last host address (since we know broadcast address; also not sure if last digit should be 63 or 62): 172.31.219.63

We could also tell first host address (172.31.219.33) and network address (172.31.219.32).

I don't understand two things:

  1. Why is first host address X.X.X.33? It is like the router is already host?
  2. Why is network address X.X.X.32? In other way, why it is not X.X.X.33 (from router address)?
  • 1
    On points 2 and 3: The broadcast address is 172.31.219.63, making the last host address 172.31.219.62. 172.31.219.64 would be the network address of the next subnet (X.X.X.64-X.X.X.95). – Tyler W Jan 7 '15 at 20:38
4

Why is first host address X.X.X.33?

  1. It is like the router is already host?
  2. Why is network address X.X.X.32? In other way, why it is not X.X.X.33 (from router address)?

The reason for calculating network addresses and host addresses like this is specified in RFC 950 and RFC 1878; contiguous binary network masks are used to divide IPv4 addresses into host portions and network portions; if the host portion is all-zeros, that is by-definition a network, and not a host.

10101100.00011111.11011011.00100001    => IP 172.31.219.33 as binary
11111111.11111111.11111111.11100000    => 27-bit network mask (binary AND with IP for net)
                              ^^^^^
                                Host bits are zero in a network mask
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
             Network bits are ones in a network mask
----------------------------------------------------------------------------
10101100.00011111.11011011.00100000    <= 172.31.219.32 is the *network*

If you need help understanding how to calculate these correctly, we have many Q&As which already respond to this problem.

-2

The first address of each subnet may be the broadcast address.

"WAIT", I hear you scream. "I thought the LAST address of each subnet was the broadcast address!"

Yeah, that is what they teach you, isn't it? "They" just expect people to be trusting and follow whatever rules people teach.

Well, the secret is: it depends on what equipment you're using. Some older equipment actually used the first address as the broadcast address. (This may be from back in the days when other technologies, like perhaps Token Ring instead of Ethernet, were competing with the standards that are more commonly used today.) People started treating the first address as a "Network ID", perhaps in part because they were trying to avoid using that address. People spread the advice to treat that as "unusable". The common reason is just that it was a "network ID". And the results seemed positive: as long as people follow that advice, people managed to avoid conflicting with that old equipment. And eventually, some networking hardware/software may have started to make the assumption that people follow that recommended advice. So rules were enforced to follow that recommended advice, which means that now even more equipment (including some newer stuff) may not be able to use such addresses. And so now, the result is that even with modern standards, instead of having only 75% of IPv4 addresses in a /30 being recognized as being usable, we have 50% available. Instead of 25% waste, we have 50% waste.

I cover this, perhaps in more detail at: ][CyberPillar][ glossary: Network IDNote 1 and/or the glossary entry for Broadcast address. (For instance, even more RFCs are referenced there.)

Hopefully that helped to explain why.


Note 1 Full disclosure, I started the ][CyberPillar][ site

  • 2
    Is this really adding anything, or a veiled attempt to drive traffic to your site? (the whole all-zero broadcast has been hashed and rehashed several times) – Ricky Beam Jan 6 '15 at 2:40
  • I actually felt I added a minor but distinct point. At the time, Mike's answer did not show the Q&A. What I answered is the why portion of point number 2 that the question asked. That's my simple answer to your publicly asked question. If you wish to discuss further, let's not pollute this question with further discussion that's really off-topic of the question. <A HREF="superuser.com/questions/612510/putty-not-sending-esc/… comment</A> is already addressing my posts. – TOOGAM Jan 6 '15 at 22:34
  • 1
    So that's "pimping your site" then. The SE way is post a complete answer, not a bunch of rambling with a link to your own site. (sites come and go, content moves around, etc. links are good, but they should quote any relevant content for the day the link rots.) – Ricky Beam Jan 7 '15 at 3:04

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