The routers of the Internet would still regard all the nodes on the three subnets as being located on the address prefix 157.60.0.0/16. The Internet routers would be unaware of the subnetting being done to 157.60.0.0/16 and therefore require no reconfiguration. The subnetting of an address prefix is not visible to the routers outside the network being subnetted.
When you assign IPv4 address prefixes in the form of subnet prefixes to the subnets of your organization, you should begin with one or more public address prefixes assigned by the Internet Corporation for Assigned Names and Numbers (ICANN) or an Internet service provider (ISP), the private address space (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16), or both. The set of starting address prefixes represent a fixed address space.
You can divide the variable portion of an IPv4 address prefix to represent additional subnets and the host IDs on each subnet. For example, the IPv4 address prefix 131.107.192.0/18 has 18 fixed bits (as the prefix length shows) and 14 variable bits (the bits in the host ID portion of the address prefix). You might determine that your organization needs up to 50 subnets. Therefore, you divide the 14 variable bits into 6 bits, which you will use to identify subnets (you can express up to 64 subnets with 6 bits) and 8 bits, which you will use to identify up to 254 host IDs on each subnet. The resulting address prefix for each subnetted address prefix has a 24-bit prefix length (the original 18 bits plus 6 bits used for subnetting).
Subnetting for IPv4 produces a set of subnetted address prefixes and their corresponding ranges of valid IPv4 addresses. By assigning subnetted address prefixes that contain an appropriate number of host IDs to the physical and logical subnets of an organization’s IPv4 network, network administrators can use the available address space in the most efficient manner possible.
Before you begin IPv4 subnetting, you must determine your organization’s current requirements and plan for future requirements. Follow these guidelines:
Determine how many subnets your network requires. Subnets include physical or logical subnets to which hosts connect and possibly private wide area network (WAN) links between sites.
Determine how many host IDs each subnet requires. Each host and router interface running IPv4 requires at least one IPv4 address.
Based on those requirements, you will define a set of subnetted address prefixes with a range of valid IPv4 addresses for each subnetted address prefix. Your subnets do not all need to have the same number of hosts; most IPv4 networks include subnets of various sizes.
Although the concept of subnetting by using host ID bits is straightforward, the actual mechanics of subnetting are a bit more complicated. Subnetting requires a three-step procedure:
Determine how many host bits to use for the subnetting.
Enumerate the new subnetted address prefixes.
Enumerate the range of IPv4 addresses for each new subnetted address prefix.