Category Archives: Subnetting

Subnetting

Subnet Cheat Sheet

 

 Subnet Cheat Sheet
Subnetting

20 Subnetting Questions and Answers from subnettingquestions.com

Question 1: What is the first valid host on the subnetwork that the node 172.20.182.215 255.255.255.128 belongs to?

Answer: 172.20.182.129

Question 2: Which subnet does host 172.21.112.211/23 belong to?

172.21.112.0

Question 3: What is the first valid host on the subnetwork that the node 10.245.110.177/20 belongs to?

Answer: 10.245.96.1


Question 4: What is the broadcast address of the network 172.16.144.0 255.255.248.0?

Answer: 172.16.151.255

Question 5: Which subnet does host 192.168.210.125/30 belong to?

Answer: 192.168.210.124

Question 6: Which subnet does host 172.27.239.230 255.255.240.0 belong to?

Answer: 172.27.224.0

Question 7: Which subnet does host 172.27.161.115 255.255.255.240 belong to?

Answer: 172.27.161.112

Question 8: What valid host range is the IP address 172.25.155.112 255.255.255.0 a part of?

Answer: 172.25.155.1 through to 172.25.155.254

Question 9: Which subnet does host 172.23.210.182/21 belong to?

Answer: 172.23.208.0

Question 10: What is the last valid host on the subnetwork 10.5.208.0/20?

Answer: 10.5.223.254

Question 11: What is the first valid host on the subnetwork that the node 192.168.253.130/28 belongs to?

Answer: 192.168.253.129

Question 12: You are designing a subnet mask for the 172.24.0.0 network. You want 70 subnets with up to 300 hosts on each subnet. What subnet mask should you use?

Answer: 255.255.254.0

Question 13: What is the first valid host on the subnetwork that the node 172.25.118.106 255.255.255.0 belongs to?

Answer: 172.25.118.1

Question 14: Which subnet does host 172.26.32.39/25 belong to?

Answer: 172.26.32.0

Question 15: What is the broadcast address of the network 192.168.202.112 255.255.255.248?

Answer: 192.168.202.119

Question 16: What is the broadcast address of the network 172.25.100.0 255.255.255.0?

Answer: 172.25.100.255

Question 17: What is the last valid host on the subnetwork 172.19.222.112 255.255.255.240?

Answer: 172.19.222.126

Question 18: How many subnets and hosts per subnet can you get from the network 172.26.0.0 255.255.255.128?

Answer: 512 subnets and 126 hosts

Question 19: What is the first valid host on the subnetwork that the node 192.168.51.193/26 belongs to?

Answer: 192.168.51.193

Question 20: What is the first valid host on the subnetwork that the node 172.30.181.215/23 belongs to?

Answer: 172.30.180.1

Subnetting

3 Easy Steps Subnetting

By David Rupu Xiao CCIE #24177


Simple, easy, fast way of the CCNA subnet training

 3 Easy Steps Subnetting
Question:

If PC1’s ip address is : 10.1.1.149/30 or 10.1.1.149 255.255.255.252

what could be PC2’s ip address?

Answer:

Step 1: What is the SIZE of each subnet ?

we have 2 bits for host parts, so the SIZE of each subnet is 4

Step 2: List all the subnets, the SIZE of each subnet is 4

subnet: 10.1.1.0 ~ 10.1.1.3

subnet: 10.1.1.4 ~ 10.1.1.7

subnet: 10.1.1.8 ~ 10.1.1.11

subnet: 10.1.1.12 ~ 10.1.1.15

subnet: 10.1.1.140 ~ 10.1.1.143

subnet: 10.1.1.144 ~ 10.1.1.147

subnet: 10.1.1.148 ~ 10.1.1.151 ( PC1 10.1.1.149 is here )

subnet: 10.1.1.152 ~ 10.1.1.155

Step 3: Get PC2’s address

We must put PC2 in the same subnet as PC1

Three IPs we can not use for PC2:

We can not use 10.1.1.148, which is the first ip –> the Network ID

We can not use 10.1.1.149, which is used by PC1

We can not use 10.1.1.151, which is the last ip –> the Broadcast address

Any other ip in the range 10.1.1.148 ~ 10.1.1.151 is good for PC2

—————————————–

Question:

If PC1’s ip address is : 10.1.1.149/29 or 10.1.1.149 255.255.255.248

what could be PC2’s ip address ?

Step 1: size is 23 = 8

Step 2: subnets list

10.1.1.0 ~ 10.1.1.7

10.1.1.8 ~ 10.1.1.15

10.1.1.148 ~ 10.1.1.155 10.1.1.149 falls in this range !

10.1.1.156 ~ 10.1.1.163

Step 3:

PC2 ‘s IP could be any of: 10.1.1.150 ~ 10.1.1.154

—————————————–

Question:

If PC1’s ip address is : 10.1.1.149/28 or 10.1.1.149 255.255.255.240

what could be PC2’s ip address ?

Step 1: size is 24 = 16

Step 2: subnets list

10.1.1.0 ~ 10.1.1.15

10.1.1.16 ~ 10.1.1.31

10.1.1.128 ~ 10.1.1.143

10.1.1.144 ~ 10.1.1.159 10.1.1.149 falls in this range !

Step 3:

PC2 ‘s IP could be any of: 10.1.1.145 ~ 10.1.1.158 except 10.1.1.149

—————————————–

Question:

If PC1’s ip address is : 10.1.1.149/27 or 10.1.1.149 255.255.255.224

what could be PC2’s ip address ?

Step 1: size is 25 = 32

Step 2: subnets list

10.1.1.128 ~ 10.1.1.159 10.1.1.149 falls in this range !

Step 3:

PC2 ‘s IP could be any of: 10.1.1.129 ~ 10.1.1.158 except 10.1.1.149

—————————————–

Question:

If PC1’s ip address is : 10.1.1.149/26 or 10.1.1.149 255.255.255.192

what could be PC2’s ip address ?

Step 1: size is 26= 64

Step 2: subnets list

10.1.1.128 ~ 10.1.1.191 10.1.1.149 falls in this range !

Step 3:

PC2 ‘s IP could be any of: 10.1.1.129 ~ 10.1.1.190 except 10.1.1.149

—————————————–

Question:

If PC1’s ip address is : 10.1.1.149/25 or 10.1.1.149 255.255.255.128

what could be PC2’s ip address ?

Step 1: size is 27= 128

Step 2: subnets list

10.1.1.128 ~ 10.1.1.255 10.1.1.149 falls in this range !

Step 3:

PC2 ‘s IP could be any of: 10.1.1.129 ~ 10.1.1.254 except 10.1.1.149

—————————————–

Question:

If PC1’s ip address is : 10.1.1.149/24 or 10.1.1.149 255.255.255.0

what could be PC2’s ip address ?

PC2 ‘s IP could be any of: 10.1.1.1 ~ 10.1.1.254 except 10.1.1.149

Given ip address: 149.149.149.149/23

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.254.0 Ip address Range: 149.149.148.0 ~ 149.149.149.255

—————————————–

Given ip address: 149.149.149.149/22

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.252.0 Ip address Range: 149.149.148.0 ~ 149.149.151.255

—————————————–

Given ip address: 149.149.149.149/21

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.248.0 Ip address Range: 149.149.144.0 ~ 149.149.151.255

—————————————–

Given ip address: 149.149.149.149/20

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.240.0 Ip address Range: 149.149.144.0 ~ 149.149.159.255

—————————————–

Given ip address: 149.149.149.149/19

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.224.0 Ip address Range: 149.149.128.0 ~ 149.149.159.255

—————————————–

Given ip address: 149.149.149.149/18

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.192.0 Ip address Range: 149.149.128.0 ~ 149.149.191.255

—————————————–

Given ip address: 149.149.149.149/17

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.128.0 Ip address Range: 149.149.128.0 ~ 149.149.255.255

—————————————–

Given ip address: 149.149.149.149/16

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.255.0.0 Ip address Range: 149.149.0.0 ~ 149.149.255.255

—————————————–

Given ip address: 149.149.149.149/15

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.254.0.0 Ip address Range: 149.148.0.0 ~ 149.149.255.255

—————————————–

Given ip address: 149.149.149.149/9

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 255.128.0.0 Ip address Range: 149.128.0.0 ~ 149.255.255.255

—————————————–

Given ip address: 149.149.149.149/2

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 192.0.0.0 Ip address Range: 128.0.0.0 ~ 191.255.255.255

—————————————–

Given ip address: 149.149.149.149/1

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 128.0.0.0 Ip address Range: 128.0.0.0 ~ 255.255.255.255

—————————————–

Given ip address: 149.149.149.149/0

What is the subnet mask ?

What is the ip address range for this subnet ?

Answer: subnet mask: 0.0.0.0 Ip address Range: 0.0.0.0 ~ 255.255.255.255

Subnetting

Subneting and Summarization

Subneting

The process of extending the default subnet mask creates a counting range in the octet that the subnet was extended into, which can be used to represent subnetworks. This allows a single Class A, B, or C network to be subdivided into many smaller groups with each group, or subdivision treated as if it were a network itself. Thus, when we extend the default Class B subnet mask to 255.255.240.0, we do so by extending the subnet mask by 4 bits into the third octet. The number of bits that the subnet mask is extended by represents a counting range for counting the number of subnetworks that new subnet mask can support, using the 2n-2 formula. Thus, the subnet mask 255.255.240.0 subnet mask can support 14 subnets (24-2). In other words, the 65,534 hosts supported by the default subnet mask can now be divided among 14 subnetworks. The number of IP addresses supported by each subnet is called an address range. To calculate the range of addresses for each subnet, we would take the decimal value for the last bit used for the subnet mask as the starting point for the first address in our subnetwork, and then increment that number for each subsequent subnet. In this octet the bit range would be 111100000. The last bit in the subnet mask would thus have a decimal value of 16 (000100000). Therefore the first IP address in the first subnet address range would be 140.12.16.1.

The address ranges for the 14 subnets would be:

• 140.12.16.1 to 140.12.31.254 • 140.12.128.1 to 140.12.143.254

• 140.12.32.1 to 140.12.47.254 • 140.12.144.1 to 140.12.159.254

• 140.12.48.1 to 140.12.63.254 • 140.12.160.1 to 140.12.175.254

• 140.12.64.1 to 140.12.79.254 • 140.12.176.1 to 140.12.191.254

• 140.12.80.1 to 140.12.95.254 • 140.12.192.1 to 140.12.207.254

• 140.12.96.1 to 140.12.111.254 • 140.12.208.1 to 140.12.223.254

• 140.12.112.1 to 140.12.127.254 • 140.12.224.1 to 140.12.239.254

Note: The IP address range for each subnet begins with a 1, as in 140.12.16.1 or 140.12.32.1 and not 140.12.16.0 or 140.12.32.0 as this would be the first address in the subnetwork, and would therefore be the network address. Similarly, the last address in the range ends in 254 and not 255 as the last address would be the broadcast address.

Summarization

Summarization allows the representation of a series of networks in a single summary address. At the top of the hierarchical design, the subnets in the routing table are more generalized. The subnet masks are shorter because they have aggregated the subnets lower in the network hierarchy. These summarized networks are often referred to as supernets, particularly when seen in the Internet as an aggregation of class addresses. They are also known as aggregated routes. The summarization of multiple subnets within a few subnets has several advantages. These include: reducing the size of the routing table; simplifying the recalculation of the network as the routing tables are smaller; network overhead scalability; and hiding network changes.

Automatic Summarization

All routing protocols employ some a type of summarization. RIP and IGRP automatically summarize at the NIC or natural class boundary as the subnet mask is not sent in the routing updates. When a routing update is received, the router checks if it has an interface in the same class network. If it has one, it applies the mask configured on the interface to the incoming routing update. With no interface configured in the same NIC network, there is insufficient information and the routing protocol uses the first octet rule to determine the default subnet mask for the routing update.

Manual Summarization

Both EIGRP and Open Shortest Path First (OSPF) send the subnet mask along with the routing update. This feature allows the use of VLSM and summarization. When the routing update is received, it assigns the subnet mask to the particular subnet. When the routing process performs a lookup, it searches the entire database and acts on the longest match, which is important because it allows for the granularity of the hierarchical design, summarization, and discontiguous networks.

A discontiguous network is a network in which a different NIC number separates two instances of the same NIC number. This can happen either through intentional design or through a break in the network topology. If the network is not using a routing protocol that supports VLSM, this will create a routing problem because the router will not know where to send the traffic. Without a subnet mask, a routing protocol that supports VLSM resolves the address down to the NIC number, which appears as if there is a duplicate address. This will incorrectly lead to the appearance of intermittent connectivity symptoms.

If there are discontiguous networks in the organization, it is important that summarization is turned off or not configured. Summarization may not provide enough information to the routing table on the other side of the intervening NIC number to be capable of appropriately routing to the destination subnets, especially with EIGRP, which automatically summarizes at the NIC boundary. In OSPF and EIGRP, manual configuration is required for any sophistication in the network design. However, because EIGRP can perform summarization at the interface level, it is possible to select interfaces that do not feed discontiguous networks for summarization.

If summarization is not possible, you can either turn summarization off and understand the scaling limitations that have now been set on the network, or you can readdress the network.

Subnetting

Tip To Remember Subnetting

Got this off another site but it’s useful here also:

To remember the subnetting tables all you have to do is start with “4” and double it until you get to “16384” Write them downward on a sheet of paper and when you are done just subtract 2 from each number.

ie:
4 = 2
8 = 6
16 = 14
32 = 30
64 = 62

once you have done that all you need to do is reverse the order of all the numbers going back up the sheet:

subnets hosts
2 62
6 30
14 14
30 6
62 2

See how the numbers flip flop between each column? My example is for class C but it works for class B just the same.

Once you have the subnet/host numbers written out, just remember the following numbers .192, .224, .240, .248, .252
class C:
sub hosts
.192 /26 2 62
.224 /27 6 30
.240 /28 14 14
.248 /29 30 6
.252 /30 62 2

The numbers with a slash (ie /26) are just short hand ways of writing out subnets. They can be really confusing if you are trying to learn subnetting for the first time. Just rember that ip addresses are made up of 32 bit addresses, or /32. These 32 bit addresses are broken down into class A,B, and C. class B are from /18 to /30 and class C go from /26 to /30. The reason the numbers don’t go up to /32 are because it goes against the rules of subnetting (according to Cisco), I don’t have any other reason why.
You need to memorize this stuff!!! When I went to work I jotted notes all over my desk and tool boxes just so I would see it all the time.

Here is the class B example:
(1). start with 4 and double it till 16384: 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384.

(2). subtract 2 from each number: 2, 6, 14, 30, 62, 126, 254, 510, 1022, 2046, 4094, 8190, 16382.

(3). write them downward on a sheet of paper and then write them back up in reverse order:
2 16384
6 8190
14 4094
30 2046
62 1022
126 510
254 254
510 126
1022 62
2046 30
4094 14
8190 6
16382 2

(4) Finally you just have to add the net number to your list… Rember these numbers: .192.0 (/1) .224.0 (/19) .240.0 (/20) .248.0 (/21) .252.0 (/22) .254.0(/23) .255.0 (/24) .255.128 (/25) .255.192 (/26) .255.224 (/27) .255.240 (/28) .255.248 (/29) .255.252. (/30)