3.2 IPv4 Dotted Decimal and Binary Basics

Reading IPv4 addresses correctly

Cisco expects CCST Networking candidates to identify IPv4 addresses and subnet formats. IPv4 addresses are 32-bit values. Humans usually write them in dotted decimal notation: four decimal numbers separated by periods, such as 192.168.10.25. Each decimal number is an octet, which means 8 bits. Because 8 bits can represent 256 values, each octet can be from 0 through 255. An address such as 192.168.10.300 is not a valid IPv4 address because 300 does not fit in one octet.

Dotted decimal is a display format, not how devices make routing decisions. Devices compare bits. The decimal octet 192 equals binary 11000000; 168 equals 10101000; 10 equals 00001010; and 25 equals 00011001. A technician does not need to convert every address by hand during daily support, but knowing the bit basis makes masks and CIDR much less mysterious.

A subnet mask is also a 32-bit value. In dotted decimal, common masks include 255.255.255.0, 255.255.0.0, and 255.255.255.128. The binary 1 bits in the mask mark the network portion of the address. The binary 0 bits mark the host portion. With address 192.168.10.25 and mask 255.255.255.0, the first 24 bits identify the network, so the network is 192.168.10.0; the last 8 bits identify the host, so .25 is one host on that subnet.

The address alone is incomplete. 10.1.2.3 could be in subnet 10.1.2.0/24, 10.1.0.0/16, 10.0.0.0/8, or a smaller custom subnet depending on the mask. This is why IP configuration screens show address, mask or prefix length, default gateway, and DNS servers. If a user reports only the address, ask for the mask or prefix length before deciding whether two hosts should be local to each other.

Some host values have special meanings inside a subnet. In traditional IPv4 subnetting, the all-zero host portion identifies the network address, and the all-one host portion identifies the directed broadcast address. In 192.168.10.0/24, 192.168.10.0 is the network address and 192.168.10.255 is the broadcast address. Usable host addresses run from 192.168.10.1 through 192.168.10.254. A host should not normally be configured with the network or broadcast address.

A practical way to troubleshoot is to compare address parts against the mask. Suppose a printer is 192.168.1.50/24 and a laptop is 192.168.2.60/24. With a /24 mask, the first three octets are the network portion. 192.168.1.0 and 192.168.2.0 are different networks, so the laptop needs a router to reach the printer. If the laptop has no gateway or the gateway is wrong, same-switch cabling will not fix that Layer 3 problem.

Also notice whether an address was typed, learned from DHCP, or assigned automatically. A transposed digit can place a device in the wrong subnet while still looking normal at a glance. A mask typed as 255.255.0.0 instead of 255.255.255.0 can make a host believe far more destinations are local, causing failed ARP attempts instead of traffic being sent to the gateway. DHCP leases usually reduce those mistakes, but a wrong DHCP scope can distribute the same bad mask to many clients at once.

Be careful with visual similarity. 172.15.10.5 is not in private range, but 172.16.10.5 is. 192.169.1.10 is not in the common private 192.168.0.0/16 range. 255.255.255.0 is a mask, not a host address. When documenting tickets, record the exact address, mask or CIDR prefix, gateway, and whether the address came from DHCP or static configuration.

Study Checkpoint

• Topic: 3.2 IPv4 Dotted Decimal and Binary Basics.
• Verify the official Cisco concept before memorizing a shortcut.
• Practice the technician action: observe, document, test, fix when supported, or escalate.