Questions tagged [subnet]

Questions relating to IP subnetting, including planning, calculation and best practices.

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227
votes
6answers
436k views

The slash after an IP Address - CIDR Notation

What does the "/16" means in here : "192.168.0.0/16", for example.
123
votes
6answers
760k views

How do you calculate the prefix, network, subnet, and host numbers?

Example: IP: 128.42.5.4 In binary: 10000000 00101010 00000101 00000100 Subnet: 255.255.248.0 How could you determine the prefix, network, subnet, and host numbers?
37
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13answers
65k views

Difference between Subnet and VLAN?

You often hear the terms Subnet and VLAN used interchangeably. With the ubiquitous nature of IP these days, when are the two not considered roughly the same from a high-level, understanding that ...
23
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8answers
19k views

Why can't devices on different VLANs, but on the same subnet, communicate?

I have a question about switching. I have two devices connected to a switch with IP addresses 192.168.5.20 and 192.168.5.10. Both devices have the same prefix, /24. That means they are on the same ...
21
votes
8answers
18k views

What is the purpose of network address?

What is the purpose of network address? (aka subnet address, starting address of each subnet) For example in this network: 192.168.1.0/24 Why we should leave 192.168.1.0 address free?
20
votes
2answers
5k views

What is the difference between 0.0.0.0 and a loopback IP address?

From the book Computer Network: The IP address 0.0.0.0, the lowest address, is used by hosts when they are being booted. It means ‘‘this network’’ or ‘‘this host.’’ ... All addresses of the ...
19
votes
1answer
7k views

What is the practical use of a /31 CIDR or 255.255.255.254?

With a /31 subnet, we are left with zero usable hosts. The /31 subnet only has two hosts - one for network and another for broadcast. I am wondering why would someone ever use this subnet.
17
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4answers
70k views

Confusing A, B, C network classes

I'm studying IPv4 addresses and came across this whole thing about classful addressing. I get the idea behind it, bit there is something I find confusing: There are two "ABC" ranges: First one: A: ...
16
votes
4answers
5k views

Why is 10.1.255.255 an invalid broadcast address?

I’ve been following the CCENT official certification book(100-105) and came upon this question in the “do I know this already?” quiz. The books only covered /24 subnetting only so far. Which of the ...
16
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4answers
58k views

Bad Mask for IP Address Error

I'm not too clear on subnetting rules and experienced an error when I tried to assign 148.26.1.176/28 to an interface on my Cisco router. Can someone please explain why it was not allowing me to ...
15
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2answers
3k views

Why are there 3 ranges of private IPv4 addresses?

Why are there three separate ranges of private IPv4 addresses of different sizes? Why not just reserve the largest (10.0.0.0 - 10.255.255.255) range and let people create /16 or /24 (or whatever) ...
14
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4answers
5k views

/31 Point-to-point bitmasks

When is it appropriate to use a /31 network in production, and is using them considered good practice? On a point-to-point link, broadcasts shouldn't be required, so is there any compelling case for ...
11
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2answers
266 views

IPv6 Subnetting - Block RA on Switches?

I'm currently planning a v6 only small datacenter network. Imagine for now one deployment consisting of 6 racks with 12 servers per rack. Each server has 2 NICs for two physically separate networks (...
10
votes
4answers
2k views

Using IP subnet at multiple datacenters

We control IPv4 and IPv6 subnets. Now we'd like to use a part of a /24 IPv4 subnet at one datacenter and another part at another datacenter. I know that announcing the subnet on the internet at ...
10
votes
4answers
22k views

Using IP addresses from IP pool given by ISP

My ISP says that it can provide an optic fiber connection with a pool of 4 static IP addresses (subnet 255.255.255.248 i think). My question is that if I get that connection, 1 ip will be used for ...
10
votes
4answers
31k views

VRFs, VLANs and subnets: difference

I have a basic understanding of VRFs, VLANs and subnets. I understand that VLANs operate on L2, and subnets and VRFs (lite) on L3. What I don't understand, is why you would choose one over the other ...
10
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3answers
4k views

3 subnets, 2 OSPF areas - Will This Work?

Will any issues occur if I have a topology involving 3 subnets and two OSPF areas where one subnet is in AREA 0 and the other two subnets are both in AREA 1? For example: [area 1, subnet 1]---[ABR #...
9
votes
2answers
2k views

Is there anything stopping me from using Class A addresses on my own network?

I've been reading about IPv4, networking, subnetting and the different classes of addresses. I understand that IP addresses beginning with numbers from 1 to 126 are considered "Class A" addresses. ...
9
votes
2answers
3k views

What are common sizes to split a /29 - /32 IPv6 subnet?

As an LIR, you are getting a /29 to /32 network allocation by the RIPE, but I am a bit overwhelmed by the size of this subnet, and don't find a starting point to split this into common sizes. What we ...
9
votes
6answers
6k views

Why do I need to specify a subnet mask on interface?

When assigning an IP address to an interface, why do I need to specify a subnet mask? I understand the purpose of subnet masks in a routing table for example (to specify a range of IP addresses as ...
9
votes
1answer
1k views

What happens when a subnet reaches capacity?

Other than not being able to claim any more IP addresses in a subnet, what other bad things happen when a subnet reaches 100% utilization, if anything?
9
votes
3answers
6k views

Generate list of subnets on switch using SNMP?

Is anyone familiar with an approach to generate a list of subnets on a particular switch (let's pick a more popular one - Cisco) using SNMP? How about a list of VLANs? If so, can you point me to the ...
8
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2answers
3k views

What is IP squat space

I came across this article and I'm trying to understand what is going on here, I found some packets on my network containing 22.0.0.0/8 and 29.0.0.0/8 IP range and It's based on: https://en.wikipedia....
8
votes
4answers
5k views

Why do I get “% Warning: use /31 mask on non point-to-point interface cautiously” when I use a /31 subnet mask?

Why do I get % Warning: use /31 mask on non point-to-point interface cautiously when I use a /31 subnet mask over Ethernet, and why am I not able to ping from R1 to R2 when both the interfaces have ...
8
votes
1answer
8k views

What is the correct mask to use with 224.0.0.0?

What is the best mask to use to allow computers to use the multicast IP address 224.0.0.0? I've only seen IP addresses using the lower 8 bits, in my case I just see 22 and 251. So my firewall uses ...
8
votes
4answers
19k views

How to divide an IPv4 network into subnets of different sizes as efficiently as possible? [closed]

I need to divide a network 172.16.0.0/12 into 3 different subnets for different purposes. One of them needs 6 host addresses, another 85 host addresses and the last 100 host addresses. It should be ...
7
votes
1answer
55k views

How to divide network into subnets?

I've been trying to figure out how to divide a network number into particular subjects where one of them require a particular number of hosts. I have the Computer Networking: A Top Down Approach but ...
7
votes
2answers
2k views

Subnetting aggregation

I saw this example online . 192.168.12.0/23 applies the network mask 255.255.254.0 to the 192.168 network, starting at 192.168.12.0. This notation represents the address range 192.168.12.0 - ...
7
votes
1answer
396 views

Is the first address of the subnet a valid configurable address in case of IPv4 and IPv6 point to point networks?

Am I correct, that first address of the subnet is a valid addresses to configure in hosts/routers in case of IPv4 and IPv6 point to point networks? For example, in case of 192.0.2.0/31 one of the ...
7
votes
3answers
163 views

Is IP address Subnet Renumbering needed here?

We have a router with NAT enabled on our edge. The NAT inside is 10.0.0.0/24. Now, we're running out IP blocks and we wish to change this to 10.0.0.0/22. We can do this quickly on the NAT router. But ...
7
votes
3answers
8k views

"Bridge” two subnets together?

Here is what the network looks like: Network A is 192.168.116.X , subnet 255.255.255.0 Network B is 12.32.8.X , subnet 255.255.255.0 There are no routers present in either of the network schemes. ...
7
votes
1answer
1k views

Why is an IPv6 Link-Local Address a /10?

I'm currently studying for my CompTIA Network+ Exam and am a bit confused as to why the IPv6 link-local address is a /10 and not a /12 or /16. Since each character in an IPv6 address is 4-bits and ...
7
votes
1answer
10k views

What is the use of the IPv6 scope ID?

In the structure struct sockaddr_in6, the field sin6_scope_id is declared. What is the use of this member? How can we know from the IPv6 address and the prefix length the scope that it should use?
7
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2answers
134 views

VLAN clarification

I've inherited a network with approximately 200 computers and no VLANs configured. We are currently adding a system that uses multicasting to communicate with each other and will be adding VoIP in 2 ...
7
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3answers
17k views

Routing traffic between two subnets

I currently admin a small network, it has two separate sites (site1 site2). Each site has independent internet, and a Sonic NSA 220 Firewall. There is currently VPN connecting the two Sonic firewalls, ...
6
votes
3answers
32k views

Setting subnet mask to 255.255.255.255

If I want just 1 device to be on the network, can I set the subnet mask to 255.255.255.255? For example, the consider the network card configuration IP 10.0.0.10 mask 255.255.255.255 ...
6
votes
2answers
5k views

What would happen to Ping if two PCs have same IP address with same Subnet Mask on same network?

If there are two PCs with same IP address on a LAN, what would happen if I ping that IP address from another PC on the same network and why?
6
votes
4answers
429 views

60 devices on a /29 network

My company asked me to set up a network with 60 devices (all have MAC addresses and use IPs, but they are not all computers), there is a 3rd party company that will maintain the network. They are ...
6
votes
3answers
2k views

CIDR and VLSM what are the differences?

CIDR improves the allocation of IP addresses. VLSM allows to define a subnet as we wish. Can CIDR used without using VLSM?
6
votes
3answers
852 views

Why couldn't a nework address be used as as broadcast address as well?

I am taking a refresher on Networking and I found out that: Network address is an IP address with all 0s after the subnet mask Broadcast address is an IP address with all 1s after the subnet mask ...
6
votes
4answers
6k views

Does a subnet mask indicate the class of a Private IP?

This question is fairly basic to those in this community... Is 192.168.X.X/24 (255.255.255.0) a class C private address? If so, can someone explain how is it possible to have a subnet mask greater ...
6
votes
2answers
626 views

VLAN-Subnets 1:1?

I often hear people saying that each VLAN is a subnet (or is associated with a subnet) but is that really true? Assume I have 10 big offices. I will have a subnet for each office but only 2 VLANs ...
6
votes
1answer
1k views

subnetting is incorrect in show ip route output

in one of the CCNA courses theres the following image taken from a routing table: I'm not very confident with my subnetting skills, so please confirm if I'm wrong: 172 is a class B network (/16). So ...
6
votes
3answers
301 views

/23 PI space - breakup

a client currently has a /23 PI space where the route-object is currently managed by ISP-A we want to split the PI space into two /24's but not certain on how best to go about this. ISP-A will use ...
5
votes
5answers
3k views

How does a host in one subnet know to forward the packet to the router MAC, when trying to send a packet to another subnet

I am trying to understand the ARP protocol and I could not understand the above. Consider the topology as follows: H1(1.1.1.1) - (1.1.1.0)R(2.2.2.0) - H2(2.2.2.2) My understanding is that if the ...
5
votes
3answers
3k views

Is a router always necessary to link two subnets?

I am thinking especially of VLANs, which represent separate subnets that may need only one switch.
5
votes
4answers
13k views

What is the purpose of subnetting, if we have NAT?

As I know, NAT is the system, that divides IP addresses into two categories - public and private. Public represents the local network from the outside and private represents the device from the inside....
5
votes
2answers
19k views

How to aggregate a routing table given IP addresses and their subnet masks

Novice networker here. I can't seem to find any straightforward answers online. Example: IP SUBNET NEXT HOP > 129.10.112.0 255.255.255.0 R1 > 129.10.80.0 255....
5
votes
6answers
4k views

packet routing between subnets on same vlan

I am trying to understand how packets are routed within the same VLAN. I understand that typically there should be a 1:1 mapping between VLANs and Subnets. However if i deploy the following ...
5
votes
2answers
849 views

Does IPv6 have similar concept of network mask?

Does IPv6 have similar concept of network mask? How does it represent a network? In the output of ifconfig, does prefixlen 128 represent a network mask? lo: flags=73<UP,LOOPBACK,RUNNING> mtu ...

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