2.2IPv4 addresses and subnet formats

An IPv4 address is not just a string of numbers—it packs two pieces of information into one: "which network, and which device." What decides that boundary is the subnet mask, and a concise way to write it is slash notation like /24. This section covers the idea of a subnet that divides a large network into manageable units, the correspondence between mask and slash notation, the network address and broadcast address reserved in each subnet, and the broadcast domain that a router separates—using gentle examples like /24 and /25. In the CCST scope you do not need complex VLSM calculation; the goal is to be able to tell "are these two devices in the same subnet or not?"

2.2.1Subnets and broadcast domains

• A subnet is a large IP network divided into manageable smaller groups. Dividing it lets you organize by department or purpose and limits the spread of needless broadcasts. Devices within the same subnet can communicate directly without going through a router.
• A broadcast domain is the range a single broadcast reaches. One subnet is, in principle, one broadcast domain, and a switch does not divide it (within the same VLAN it passes straight through). A router (or L3 switch) is what separates broadcast domains, and a broadcast does not cross into another subnet.
• So "same subnet or different subnet" matters in practice: if the same, communication is direct; if different, it goes via the default gateway (routing). Checking whether two devices' IPs and masks share the same network portion is a basic move in isolation.

2.2.2Subnet mask and slash (CIDR) notation

• A subnet mask shows how much of an address is the network portion and where the host portion begins. Network-portion bits are 1 and host-portion bits are 0. Example: 255.255.255.0 has its first 24 bits set to 1, meaning a 24-bit network portion.
• Slash (CIDR) notation writes the mask's number of one bits directly, as in /24—a shorthand. 255.255.255.0 = /24, 255.255.255.128 = /25, 255.255.0.0 = /16. A larger number means a longer network portion and fewer hosts fit.
• Each subnet reserves two addresses that cannot be given to devices: the network address (all host bits 0—the first address, naming the subnet itself) and the broadcast address (all host bits 1—the last address, addressed to everyone in the subnet). So in a /24 (256 addresses) the number actually assignable to devices is 256 - 2 = 254.

Suppose an office asks, "why can the PCs of A-san and B-san in the same department not share files directly with each other?" You check both machines with ipconfig: A-san is 192.168.10.20 / 255.255.255.0 (that is, /24) and B-san is 192.168.11.30 / 255.255.255.0 (also /24). Understanding slash notation reveals the cause. A /24 has a network portion of the first 24 bits—the first three octets—so A-san's subnet is 192.168.10.0/24 and B-san's is 192.168.11.0/24; the third octet differs (10 vs. 11), so the two are in different subnets. Different subnets are not the same broadcast domain and cannot communicate directly; doing so requires routing via the default gateway. So the idea "just move the switch cable and it will work" is off target, and what to check is each PC's gateway setting and whether the router or L3 switch that routes between those two subnets is working. Conversely, if both were 192.168.10.x/24, sharing the same network portion, they should communicate directly, and suspicion would move to physical/L2 or a firewall. The key point: if you can tell "same subnet or different" from the IP and mask (slash notation), you can immediately isolate whether this is a direct-communication matter or a routing matter. In the CCST scope you need not master fine bit calculation; being able to reliably do this one thing—identifying same vs. different subnet—is the foundation.

2.2.3Section summary

• A subnet is a large network divided into smaller groups, and the subnet mask decides the boundary between the network portion (1) and the host portion (0)
• Slash notation is the count of one bits (255.255.255.0 = /24). Each subnet reserves two addresses—first = network, last = broadcast—so a /24 holds 254 devices
• A router (L3) separates broadcast domains. Telling same vs. different subnet from IP and mask settles whether it is direct communication or routing