02-12-2020 02:21 AM
Guys,
i need som help with subnetting.
here is something i dont understand about the subnetting part.
Lets say i got a class B address:
128.32.0.0 /16
I do know that i have 1 network and around 65000 clients.
But when i start subnetting, lets say i got this:
128.32.15.0/30
i can have muti networkaddresses and only 4 hosts. I do know why i can have only 4 hosts, but i am not sure why i can have multiple networkaddresses. Which addresses for the network can i chose between in this specific situation?
many thanks for the help.
02-12-2020 08:35 AM
I am not clear what you are really asking and whether it is really just a syntax question or whether there is something else in the question. I follow your logic that with 128.32.0.0/16 you have one network with lots of hosts. I am confused when we get to 128.32.15.0/30 and your statement is about having multi network addresses. We both agree that with this mask there are 4 IP addresses in the subnet (and typically we would say that this supports 2 hosts since the typical IP subnet has a network address and a broadcast address). I am not sure what you mean about multi network addresses. Perhaps you mean that with a mask of /30 that there are many network addresses available (because with that mask there are many individual subnets). To be specific about terminology 128.32.15.0/30 is a single small subnet and has a single network address. If you are asking something else please clarify what it is.
02-12-2020 09:00 AM - edited 02-12-2020 09:31 AM
this is called VLSM -variable length subnet mask or CIDR
128.32.15.0/30 is already subnetted and likely gave out to very small businesses by ISP. But, let's say you are very large corporation and have 128.32.0.0/16 from ISP. you must divide that by smaller chunks based on hosts in a subnet (sub-network) per several sites. All sites will get blocks (chunks) based on number of hosts but one of those already subnetted network (block) will be reserved for site-to-site or WAN-to-HQ connections where you need 2 hosts per link.
Let's use /24 for easy explanation. taking /16 and sub-dividing into /24 gives us 256 networks with 254 hosts per one. anyway, 1st subnet out of 128.32.0.0/16 will be 128.32.0.0/24, 2nd is 128.32.1.0/24, 3rd 128.32.2.0/24 ......128.32.15.0/24 . .... and so on.
Now you can take segment 128.32.15.0/24 and sub-divide further down to smaller chunks to get your WAN-to-HQ links: 128.32.15.0/30, 128.32.15.4/30, 128.32.15.8/30, 128.32.15.12/30, and so on. See, /30 out of /24 gives us 64 links from HQ to remote sites.
For example, HQ uses 28.32.0.0/24. Site A uses 28.32.1.0/24, Site B uses 28.32.2.0/24 but none of site will use 28.32.15.0/24 as this one is reserved for WANs links. Those WAN-to-HQ links are as follow:
HQ to Site A link uses 128.32.15.0/30,
HQ to Site B uses 128.32.15.4/30 ..
HQ to site C uses 128.32.15.8/30 .. and so on.
if you are small company and got 128.32.15.0/30 from ISP, you must use NAT to go on the Internet; Also, it is possible that my subnet is 128.32.15.4/30, and we both are part of larger segmentted block assigned for our region by ISP
Regards, ML
**Please Rate All Helpful Responses **
02-13-2020 11:01 AM - edited 02-13-2020 11:22 AM
I think i get it. When subnetting, the subnetted host bits can be used as new networks.
Your example:
1st subnet out of 128.32.0.0/16 will be 128.32.0.0/24, 2nd is 128.32.1.0/24, 3rd 128.32.2.0/24 ......128.32.15.0/24 . .... and so on.
With a /16 the third octet will be for hosts by design, but when subnetting /24 the third octet can be completely used for subnets.
Although this confuses me:
HQ to Site A link uses 128.32.15.0/30,
HQ to Site B uses 128.32.15.4/30 ..
HQ to site C uses 128.32.15.8/30
What will be the ipaddresses i can use in these three examples? /30 bits 30 bits from 32 are for my network. But what are my ip addresses?
02-13-2020 01:35 PM
Hope I can help resolve your confusion.
For 128.32.15.0/30 The addresses you can use are 128.32.15.1 and 128.32.15.2.
For 128.32.15.4/30 The addresses you can use are 128.32.15.5 and 128.32.15.6.
For 128.32.15.8/30 The addresses you can use are 128.32.15.9 and 128.32.15.10.
If the subnet mask is /30 then 30 bits are network/subnet and 2 bits are host addressing. With 2 bits of addressing there can be 4 addresses. One address is the network address (beginning of the range), one address is the broadcast address (last of the range) and the middle addresses are usable host addresses.
02-13-2020 03:58 PM
02-14-2020 12:24 PM
Thank you for the info. About the /31 part, if there is no broadcast adress or if a network address will be used s an ip address, will that cause any problems?
02-14-2020 02:59 PM
02-15-2020 07:14 AM
Just to expand a bit on @Joseph W. Doherty correct point that /31 is used for point to point connections. The concept of broadcast is meaningful in environments where you can send a single packet out an interface and it can be received by multiple devices. But on a point to point connection when you send a single packet out that interface it can be received by only one device and so the concept of broadcast does not apply to point to point connections.
02-14-2020 09:10 AM
Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: