~ /project-1 /subnetting
Project 1 · Subnetting

One /24 becomes four /26.

The brief gives the block 199.19.30.0/24 and a /26 mask. Two subnets go to Network A and Network B, and the other two are kept for future use.

255.255.255.1922 borrowed bitsBlock size 64

The split#

Network A · G0/0.0 /26usable .1 – .62
broadcast .63
Network B · G0/1.64 /26usable .65 – .126
broadcast .127
Future.128 /26usable .129 – .190
broadcast .191
Future.192 /26usable .193 – .254
broadcast .255

Last octet of the mask: 11000000 = 128 + 64 = 192 · 2 subnet bits · 6 host bits

The working#

Borrow 2 bits#

/26 − /24 = 2 bits taken from the host part. 2² = 4 subnets.

Mask#

The last octet becomes 11000000 = 128 + 64 = 192, so the mask is 255.255.255.192.

Hosts#

32 − 26 = 6 host bits. 2⁶ = 64 addresses, minus the network and broadcast addresses = 62 usable.

Block size#

256 − 192 = 64, so subnets start at .0, .64, .128 and .192.

2
Borrowed bits
4
Subnets
62
Usable hosts each
64
Block size

Subnet table#

Subnet Network Usable range Broadcast Use
1 199.19.30.0/26 .1 – .62 .63 Network A (R1 G0/0)
2 199.19.30.64/26 .65 – .126 .127 Network B (R1 G0/1)
3 199.19.30.128/26 .129 – .190 .191 Future use
4 199.19.30.192/26 .193 – .254 .255 Future use

Addressing plan#

The first usable address of each subnet goes to the router interface, which is the default gateway for that subnet. Hosts follow in order.

Device Interface IP address Subnet mask Default gateway
R1 G0/0 199.19.30.1 255.255.255.192 N/A
R1 G0/1 199.19.30.65 255.255.255.192 N/A
PC A NIC 199.19.30.2 255.255.255.192 199.19.30.1
PC B NIC 199.19.30.3 255.255.255.192 199.19.30.1
PC1 NIC 199.19.30.66 255.255.255.192 199.19.30.65
PC2 NIC 199.19.30.67 255.255.255.192 199.19.30.65
Web Server NIC 199.19.30.68 255.255.255.192 199.19.30.65

Quick check: same subnet?

With a /26, two addresses share a subnet only if they fall in the same block of 64. PC A (.2) is in 0–63 and PC2 (.67) is in 64–127. They are on different subnets, so PC A sends its ping to its gateway, .1, and R1 routes it.