In This Lesson We Will:
- Dissect the IPv4 Header
- Learn the IPv4 classes
- Learn the purpose of the subnet mask
- Review IPv4 Subnetting and VLSM
While IPv6 is quickly gaining popularity in networks, most networks still work with private, classful IPv4 addresses. In fact, if you open up your command prompt or terminal on your computer and type ‘ipconfig’ you’ll see your network is implementing a private IPv4 address. Additionally, the CCNA will require that you know how to subnet an IPv4 address quickly, so I’ll recap what I believe is the quickest method of IPv4 subnetting. I won’t touch on the binary just yet, because trust me when I say, it’s dry and boring. But you will need to know it. I will dedicate a section on its own to the binary behind subnetting in the next lesson.
IPv4 Header Dissected:
All IPv4 addresses have this header. It’s worth knowing what the header contains, however, the CCNA does not require that you know all aspects of the IPv4 header. Additionally, IPv4 addresses consist of “Network bits” and “Host bits” and contain both the source address and the destination IPv4 address.
Classful IPv4 Addresses:
There are five classes of IPv4 addresses. The class of the address determines the subnet mask:
Class A – First Octet range of 1-127 – 8 Network Bits – 24 Host Bits- Subnet mask 255.0.0.0
Class B – First Octet range of 128-191 – 16 Network Bits – 16 Host Bits – Subnet mask 255.255.0.0
Class C – First Octet range of 192-223 – 24 Network Bits – 8 Host Bits – Subnet mask 255.255.255.0
Class D – First Octet range of 224-239 – These are not used for addressing. Class D networks are used for Multicast addresses
Class E – First Octet range of 240-255 – These are not sued for addressing. Class E networks are reserved for Research
The subnet mask is the network bits of the classful address in binary bits. So, for a class A address of 10.0.0.0, the default subnet mask is 255.0.0.0, because there are 8 total host bits. 8 binary bits = 255. An L3 device will use the subnet mask to determine the network address for a packet by finding the bit boundary where the series of binary 1s ends and the binary 0s begin. This bit boundary for default masks is always on the octet boundary.
I’ve touched on IPv4 subnetting previously, I will link to that post here as it explains how to subnet and find the wildcard mask within seconds: