Subnet ID Explained: A Beginner’s Guide to Subnetting

The internet is often called the network of networks, as it links millions of private and public networks into one. Through a practice called subnetting, each network can be further divided into one or more smaller subnetworks, making services such as the World Wide Web possible.

The hierarchy of subnets makes communication more efficient, devices more secure, and extensive networks easier to manage. Subnets also enable various applications when setting up and using proxy servers. All these use cases start from understanding what subnets and subnet IDs are.

What Is a Subnet and Why Does It Matter?

Devices communicate with each other by sending data packets over a shared wired or wireless network. Routers and switches direct said packets based on IP addresses by following the rules of various network protocols, such as TCP/IP.

Subnetworks (subnets for short) are created when we divide a larger IP network into one or more smaller subdivisions. So, subnets are logical partitions within IP networks created to improve network management, network security, and performance.

• Subnets help find the best data routing paths by shortening the distance the traffic must travel and bypassing unnecessary nodes.
• Subnetting can increase performance in certain networks, such as those used for cloud services, as parts are dedicated to help with network management.
• Containing security threats or other issues from spreading to the whole network is easier for cybersecurity specialists and network administrators.
• IP address allocation requires subnets to ensure IPs are used efficiently. IP address allocation also impacts some proxy server use cases, such as web scraping.

What Is a Subnet ID?

Subnet IDs, or subnet identifiers, are a part of an IP address that represents a specific subdivided part of a larger IP network. Subnet IDs are frequently used interchangeably with network addresses, but they aren’t the same.

The latter is a broader identifier relating to communication between networks. Routers use subnet IDs to determine where the data packet needs to go when deciding between devices in the network. Network administrators can also use them to separate parts of the network for certain tasks.

Subnet ID is represented in the IP address itself when we combine the network prefix and the subnet portion of the IP address. Every IPv4 IP address is a 32-bit number divided into four sections called octets. For example, 192.168.1.50.

The first part of “192” of the IP address shows the network portion size, and the second part “168.1.50” is divided into a subnet and host portions depending on the subnet mask. Networks and their IP addresses were originally labeled in classes from A to E.

• Class A uses the first octet to represent the network and the octets after to specify the subnet devices (hosts) within that network. For example, the IP address 10.0.0.0 can support approximately 16 million hosts (2^24 - 2) in a subnet.
• Class B networks use the first two octets to indicate the network and subnet devices after. For example, the IP address 172.16.0.0 can support 65,534 hosts (2^16 - 2) in a subnet.
• Class C networks use the first three octets for the network and the rest for host devices. For example, the IP address 192.168.1.50 can support 254 hosts (2^8 - 2) in a subnet.

The same applies to other IPv4 address classes up to class E, but they aren’t often used in general networking use cases. The class system and IPv4 IP addresses are being replaced by IPV6 subnets .

IPv6 IP addresses are 128-bit long, written in eight groups of hexadecimal digits, and use a hierarchical system instead of classes like in IPv4. Here’s an example of an IPv6 address.

2001:0db8:85a3:0000:0000:8a2e:0370:7334

Such IP addresses have their subnet IDs in 16 bits after the routing prefix (usually, the first 48 bits). So, we first must locate the routing prefix assigned by your internet service provider. In our example IP address, this is "2001:0db8:85a3".

The following 16 bits are then subnet ID, which in this case is ":0000". Unlike in IPv4, all the remaining 64 bits can be used for subnet IDs to identify host devices on a subnet and aren't reserved for the network. So, the number of possible host addresses is much larger (2^64).

Understanding Subnet Masks and CIDR Notation

A subnet mask is a 32-bit number that divides an IPv4 IP address into its network and host portions. By indicating these parts, the mask tells us how big the network is and how many IP addresses there are.

The subnet mask of 192.168.1.50 is 255.255.255.0 since it's the default for the C class. It's the most commonly used mask, showing that there are 256 addresses using only the fourth octet.

Without additional notation, the IP address alone doesn't tell us its mask, but we can make a couple of assumptions and get the needed bits for hosts in a subnet and its mask. First, we need to convert the IP address 192.168.1.50 to binary - 11000000.10101000.00000001.00110010.

Further, suppose we need 254 IP addresses in our network. We can use the formula 2^N-2, where N stands for the number of bits reserved for the host portion of the address. Two IP addresses are subtracted because they are usually reserved for the network itself.

Since we need 254 IP addresses, we get 2^N-2=254. Solving it tells us that N is equal to 8, which means that we need 8 bits for hosts. The last 8 bits must be reserved for hosts, and the first 24 must define the network.

When we convert the 255.255.255.0 to binary, each "1" represents part of the network bits, and each "0" represents the host bits. In our case, the first three octets must be only "1" and the last one must be only "0". This is exactly what we get when we convert 255.255.255.0 to binary.

255.255.255.0 = 11111111.11111111.11111111.00000000

CIDR (Classless Inter-Domain Routing) notation, also known as slash notation, simplifies the naming scheme for subnet IDs. Using it, we would write 192.168.1.50/24, where /24 would represent that the first 24 bits are reserved for the network and the last 8 for the hosts.

More importantly, CDIR notation is the only way to note subnet IDs for IPv6 addresses. In this notation, IPv6 addresses with subnet IDs are written like this: 2001:0db8:85a3:0000:0000:8a2e:0370:7334/64, which means that the subnet allows 2^64 hosts.

How to Find a Subnet ID

The process we explained before, while educational, isn't very convenient for finding subnet IDs. Network management professionals are able to convert IP addresses to binary and calculate the formula in their heads, but it takes a lot of practice.

An easier way is to use the so-called cheat sheets for subnet IDs. We provided one for IPv4 addresses below, but to use it, you'll first need to figure out your IP address (use an IP address lookup tool if needed).

CIDR notation is used to find the subnet IDs for IPV6 addresses. In our previous example, the IP address uses "/64", which means that the first four groups are for the routing prefix and the last four are for the host part. Replacing the last four groups with zeros gives us subnet IDs for IPv6 addresses with this prefix.

2001:0db8:85a3:0000:0000:0000:0000:0000

The process gets a bit more complicated with some other prefixes and takes some practice to master. Often, it's much faster and easier to use one of the many subnet ID calculators available online. They work for both IPv4 and IPv6 addresses.

Examples of Subnetting in Action

In a real-life setting, a network administrator might need to change the number of subnets for network management purposes, for example, to isolate different departments within the network.

Suppose we have a network of 192.168.1.50/24. According to our cheat sheet or subnet IDs calculator, the mask is 255.255.255.0 or /24 in CIDR notation. Such a network has 256 IPs, with two reserved for the network itself.

Suppose further that a network administrator wants to split the network into four subnets, one for each of the HR, finance, IT, and marketing departments. In this case, he needs to borrow 2 bits from the host portion because 2^2 equals 4. According to the cheat sheet, the new subnet in CIDR notation is /26 or 255.255.255.192.

Each subnet now has 6 bits for hosts and 64 IP addresses (2^6). Since two are reserved for the network, the usable IP count per subnet is 62. For example, the first subnet will have an IP range from 192.168.1.0 to 192.168.1.63.

Now, we can understand how setting up different subnet IDs enables various network improvements.

• Reducing broadcast traffic . Smaller subnets mean that routers only need to know the subnet IDs and their ranges for each department, making the network more efficient and better performing.
• Improving network security . Different subnet IDs can have different levels of access, separate firewall rules for departments (HR doesn't need that much access compared to IT, for example), and easier reactions when there are security issues.
• Simplifies troubleshooting . If any issues in connectivity occur, they might not have an impact on the whole network. They are also easier to solve when isolated to certain departments with different subnet IDs.
• Optimizing IP allocation . Subnetting helps networks allocate the needed amount of IP addresses to each department and plan for future growth.

Resident Subnet ID, Private Subnets & VLANs

Resident subnet IDs are identifiers for a particular subnet that the IP address belongs to (or "resides" in). In other words, the resident subnet IDs correspond to the subnet that contains the given IP address.

Administrators often need to know the resident subnet IDs. It can be done by applying the subnet mask to the IP address and zeroing out the host bits. For example, 192.168.1.50/26 has a resident subnet ID of 192.168.1.0/26.

Such an operation is frequently used when managing private subnets. These are subnets that have IP ranges reserved for internal use within a private network. They should not be confused with Virtual Local Area Networks (VLANs).

VLANs can also create separate isolated networks within the same router or switch, but function on the data link layer. In practice, this means that MAC addresses are used for identification and not IPs or subnet IDs, as in subnets.

Conclusion

While subnetting can get a lot more complicated, the basic principles outlined here are enough to understand subnet IDs. Their application is vast and will give you much flexibility in network management and many proxy use cases. The rest depends on how you will implement subnets in your projects.

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