Can I use the first IP in a subnet, for instance 192.168.0.0?
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@JaredBusch said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@wrx7m said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@wrx7m said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
1990s there were still garbage services
I do remember that I was not able to change the SNM on an old HP jetdirect print server that we had connected to some dot matrix printers via centronics connectors. It was 10base-T and also had a BNC connector on it.
Yeah, 10Base-T and BNC was early 1990s. By 1997 we were already way, way past that stuff even in small, cheap deployments.
I finally got to retire that thing last year.
What? WTF was it hooked to?
They make BNC to RJ 45 adapters
This one actually had both connectors. I just used the RJ45.
Edit: It was similar to this one, but mine had 3 parallel ports.
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Is it possible / bad practice to use the first address in the network, for instance 192.168.0.0 (netmask 255.255.255.0) ?
If I remember correctly a long time ago it wasn't possible but nowadays it is. I never use it but when you have small subnets like /29 it could be nice.
@Pete-S : The short answer is No. The longer answer is that it depends on the math.
In 192.168.0.0/24, 192.168.0.0 is the network address... 192.168.0.255 is the broadcast address.
In a 192.168.0.0/23, 192.168.0.0 is the network address, and 192.168.1.255 is the broadcast address.... 192.168.1.0 is a usable IP address in that network.
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@dafyre said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Is it possible / bad practice to use the first address in the network, for instance 192.168.0.0 (netmask 255.255.255.0) ?
If I remember correctly a long time ago it wasn't possible but nowadays it is. I never use it but when you have small subnets like /29 it could be nice.
@Pete-S : The short answer is No. The longer answer is that it depends on the math.
In 192.168.0.0/24, 192.168.0.0 is the network address... 192.168.0.255 is the broadcast address.
In a 192.168.0.0/23, 192.168.0.0 is the network address, and 192.168.1.255 is the broadcast address.... 192.168.1.0 is a usable IP address in that network.
No that’s not how that works. The first address in a subnet is never usable. The last address in a subnet is never usable. There is no in between. Every single subject has exactly one first address and exactly one last address.
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@JaredBusch said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@dafyre said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Is it possible / bad practice to use the first address in the network, for instance 192.168.0.0 (netmask 255.255.255.0) ?
If I remember correctly a long time ago it wasn't possible but nowadays it is. I never use it but when you have small subnets like /29 it could be nice.
@Pete-S : The short answer is No. The longer answer is that it depends on the math.
In 192.168.0.0/24, 192.168.0.0 is the network address... 192.168.0.255 is the broadcast address.
In a 192.168.0.0/23, 192.168.0.0 is the network address, and 192.168.1.255 is the broadcast address.... 192.168.1.0 is a usable IP address in that network.
No that’s not how that works. The first address and a subnet is never usable. The last address and a subnet is never usable. There is no in between. Every single subject has exactly one first address and exactly one last address.
That is what he is saying. The network is 192.168.0.0, so 192.168.1.0 is usable.
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@JaredBusch said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@dafyre said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Is it possible / bad practice to use the first address in the network, for instance 192.168.0.0 (netmask 255.255.255.0) ?
If I remember correctly a long time ago it wasn't possible but nowadays it is. I never use it but when you have small subnets like /29 it could be nice.
@Pete-S : The short answer is No. The longer answer is that it depends on the math.
In 192.168.0.0/24, 192.168.0.0 is the network address... 192.168.0.255 is the broadcast address.
In a 192.168.0.0/23, 192.168.0.0 is the network address, and 192.168.1.255 is the broadcast address.... 192.168.1.0 is a usable IP address in that network.
No that’s not how that works. The first address in a subnet is never usable. The last address in a subnet is never usable. There is no in between. Every single subject has exactly one first address and exactly one last address.
He pointed that out.
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@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@JaredBusch said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@dafyre said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Is it possible / bad practice to use the first address in the network, for instance 192.168.0.0 (netmask 255.255.255.0) ?
If I remember correctly a long time ago it wasn't possible but nowadays it is. I never use it but when you have small subnets like /29 it could be nice.
@Pete-S : The short answer is No. The longer answer is that it depends on the math.
In 192.168.0.0/24, 192.168.0.0 is the network address... 192.168.0.255 is the broadcast address.
In a 192.168.0.0/23, 192.168.0.0 is the network address, and 192.168.1.255 is the broadcast address.... 192.168.1.0 is a usable IP address in that network.
No that’s not how that works. The first address in a subnet is never usable. The last address in a subnet is never usable. There is no in between. Every single subject has exactly one first address and exactly one last address.
He pointed that out.
Yes in a very confusing way by starting off the short answer is......
There is only one answer to the question
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OK, I did some more research and made some test. I believe most people got this one wrong and for reasons that are historical.
Assume we have the network 192.168.1.0/24.
Subnet mask 255.255.255.0. The address range is 192.168.1.0 to 192.168.1.255.192.168.1.0 is a valid host IP - contrary to what most people believe.
192.168.1.255 is reserved for directed broadcast.Why?
An IP like 192.168.1.0 used to be excluded from use by a host in the past. This was obsoleted in conjunction with the introduction of classless subnets, CIDR. Mentioned in 1995, RFC 1878, which also obsoleted something related, which was the exclusion of certain subnets called subnet zero and the all-ones subnet.
In the past IPs like 192.168.1.0 has also been used as a broadcast address but that practice is also obsolete. RFC 1812 (also 1995) states that 192.168.1.255 should be used for directed broadcast in the 192.168.1.0/24 network and that 192.168.1.0 is forbidden to use for that purpose.
Problem when something becomes obsolete is that you still have old equipment, old protocols and old habits in use. So it takes many years before you can actually stop doing certain things that were needed in the past.
To test the state of things today I spun up some VMs. I used 172.16.0.0/24 as my network.
No problem setting 172.16.0.0 as IP address on CentOS or Debian for example. Everything works as you would expect.
You could however see some remnants of the past, like this:
As mentioned above, it was a long time since that was considered a broadcast address.Windows 7 was however another story. You can't enter 172.16.0.0 as a valid IP address in network settings. But you can do it on the command line with netsh. And then it shows up as expected. Network works as expected too.
So all in all, it is technically OK to use the first IP as an host IP. It's not reserved anymore and hasn't been for more than two decades. Protocols that used that IP for broadcast or reserved for the network address are not in use anymore.
The biggest risk is probably to run into applications where they on purpose don't allow you to enter a specific "invalid" IP address.
That said, it would probably be very confusing for most people.
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Another fun little tidbit is that when you get to small size network such as /31 (2 IP addresses) then there are no broadcast address and of course no network address. RFC 3021 from year 2000 - for point to point links.
I wonder if Windows 7 for instance would complain in the GUI trying to enter that...
Update: Of course it didn't work in Windows 7 GUI either.
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@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@wrx7m said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@wrx7m said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
1990s there were still garbage services
I do remember that I was not able to change the SNM on an old HP jetdirect print server that we had connected to some dot matrix printers via centronics connectors. It was 10base-T and also had a BNC connector on it.
Yeah, 10Base-T and BNC was early 1990s. By 1997 we were already way, way past that stuff even in small, cheap deployments.
I finally got to retire that thing last year.
What? WTF was it hooked to?
Seriously? I had a wide cartridge dot matrix printer in use until last year that had that same HP jet direct box. The printer had a centronix connector on it. In fact I just gave the printer to a client because I couldn't sell it. Instead of just trashing it, I gave it to a client who had the same printer - so now they have a backup.
I'm guessing most of those dot matrix printers sold today have built in NICs, but even 10 years ago I could and did buy dot matrix that had parallel ports on them instead of built in NICs.
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Another fun little tidbit is that when you get to small size network such as /31 (2 IP addresses) then there are no broadcast address and of course no network address. RFC 3021 from year 2000 - for point to point links.
I wonder if Windows 7 for instance would complain in the GUI trying to enter that...
Update: Of course it didn't work in Windows 7 GUI either.
Interesting about the no broadcast addresses - at what point are broadcasts addresses dropped from CIDR notation?
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Another fun little tidbit is that when you get to small size network such as /31 (2 IP addresses) then there are no broadcast address and of course no network address. RFC 3021 from year 2000 - for point to point links.
Good point.
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@Dashrender said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Another fun little tidbit is that when you get to small size network such as /31 (2 IP addresses) then there are no broadcast address and of course no network address. RFC 3021 from year 2000 - for point to point links.
I wonder if Windows 7 for instance would complain in the GUI trying to enter that...
Update: Of course it didn't work in Windows 7 GUI either.
Interesting about the no broadcast addresses - at what point are broadcasts addresses dropped from CIDR notation?
/31
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@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Dashrender said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Another fun little tidbit is that when you get to small size network such as /31 (2 IP addresses) then there are no broadcast address and of course no network address. RFC 3021 from year 2000 - for point to point links.
I wonder if Windows 7 for instance would complain in the GUI trying to enter that...
Update: Of course it didn't work in Windows 7 GUI either.
Interesting about the no broadcast addresses - at what point are broadcasts addresses dropped from CIDR notation?
/31
Then the "when you get to a small sized network such as" doesn't really work - that would be a single listed exception, not small size. Hence my question.
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@Dashrender said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Dashrender said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Another fun little tidbit is that when you get to small size network such as /31 (2 IP addresses) then there are no broadcast address and of course no network address. RFC 3021 from year 2000 - for point to point links.
I wonder if Windows 7 for instance would complain in the GUI trying to enter that...
Update: Of course it didn't work in Windows 7 GUI either.
Interesting about the no broadcast addresses - at what point are broadcasts addresses dropped from CIDR notation?
/31
Then the "when you get to a small sized network such as" doesn't really work - that would be a single listed exception, not small size. Hence my question.
Both /31 and /32 have no broadcast address. So two single exceptions in that case.
And the broadcast address is more special than one would imagine because it's not normally used.
There seem to be two main types of L3 IP broadcasts.- Local broadcasts that uses address 255.255.255.255 (only within the subnet)
- Directed broadcast that uses the broadcast address (can be routed)
Directed broadcast however are disabled by default in routers and switches because it's has been abused in DDoS attacks called smurf attacks. The only protocol I could find that might need directed broadcast is WakeOnLan and only when you try to wake up a machine in another subnet.
As a test I tried to ping the broadcast address in a couple of different LANs and I couldn't get a response from anything. If it was enabled I would imagine that all hosts would respond to the ping.
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Both /31 and /32 have no broadcast address. So two single exceptions in that case.
Exactly. One and two usable is where it is so small that it can't. /31 and smaller.
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
And the broadcast address is more special than one would imagine because it's not normally used.
I use it manually to light up ARP tables.
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@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
And the broadcast address is more special than one would imagine because it's not normally used.
I use it manually to light up ARP tables.
Exactly how do you mean?
I thought ARP requests used layer 2 broadcast with a special mac address to get IP addresses.
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
And the broadcast address is more special than one would imagine because it's not normally used.
I use it manually to light up ARP tables.
Exactly how do you mean?
I thought ARP requests used layer 2 broadcast with a special mac address to get IP addresses.
ARP does. But when you broadcast (e.g. ping the broadcast IP) it tells every L3 service to respond, which requires their corresponding ARP entry to be updated.
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@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@scottalanmiller said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
And the broadcast address is more special than one would imagine because it's not normally used.
I use it manually to light up ARP tables.
Exactly how do you mean?
I thought ARP requests used layer 2 broadcast with a special mac address to get IP addresses.
ARP does. But when you broadcast (e.g. ping the broadcast IP) it tells every L3 service to respond, which requires their corresponding ARP entry to be updated.
Do you have a LAN with nodes where that actually works? If I ping the broadcast IP (tried several different LANs) I get zero replies.
Mix of windows, linux, bsd, printers, ip cameras, switches, OOB interfaces and god knows what.
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@Pete-S said in Can I use the first IP in a subnet, for instance 192.168.0.0?:
Do you have a LAN with nodes where that actually works? If I ping the broadcast IP (tried several different LANs) I get zero replies.
Don't need the replies, the ARP table updates
