Subnet a /24 for two LANs, then find a mistyped mask and a gateway outside its subnet

Steps

1. Plan on paper before you touch Packet Tracer. LAN A needs 100 hosts and LAN B needs 50, both from 192.168.50.0/24, with LAN A first. For each, find the fewest host bits that fit (2 to the power h, minus 2), the prefix, the mask, the block size, the network address, the broadcast address and the usable range. Give each router interface the first usable address. Then check your work against the Addressing table: LAN A is 192.168.50.0/25 (126 hosts, broadcast 192.168.50.127) and LAN B is 192.168.50.128/26 (62 hosts, broadcast 192.168.50.191). Write down what is left over: 192.168.50.192/26.
2. Place the devices and cable them as the Links table lists, with Copper Straight-Through cables throughout. Configure R1, typing LAN B's mask exactly as the change record gives it (it contains a mistake you will find in step 5):

Router> enable
Router# configure terminal
Router(config)# hostname R1
R1(config)# interface g0/0/0
R1(config-if)# ip address 192.168.50.1 255.255.255.128
R1(config-if)# no shutdown
R1(config-if)# interface g0/0/1
R1(config-if)# ip address 192.168.50.129 255.255.255.224
R1(config-if)# no shutdown
R1(config-if)# end

3. On each PC open Desktop > IP Configuration, choose Static, and enter the address, mask and gateway from the Addressing table: PC1 192.168.50.10, 255.255.255.128, gateway 192.168.50.1; PC2 192.168.50.20, 255.255.255.128, gateway 192.168.50.1; PC3 192.168.50.170, 255.255.255.192, gateway 192.168.50.129. Wait for every link light to turn green (Fast Forward Time helps).
4. Test from PC1:

C:\> ping 192.168.50.20
C:\> ping 192.168.50.170

PC2 answers, because it is in PC1's own subnet. PC3 does not: the replies come back as Destination host unreachable from 192.168.50.1, which means R1 itself has nowhere to send the packet. 5. Fault 1, a wrong mask on the router. Read R1's own view of LAN B:

R1# show ip interface brief
R1# show ip interface g0/0/1
R1# show ip route

The brief view shows the address but not the mask, so it looks fine. The detail reads Internet address is 192.168.50.129/27. Work it through: a /27 has a block of 32, so .129 sits in the block 128 to 159, and R1's connected route covers only 192.168.50.128/27. PC3's 192.168.50.170 falls outside it, and R1 has no other route that covers it. Fix the mask and read the route again:

R1# configure terminal
R1(config)# interface g0/0/1
R1(config-if)# ip address 192.168.50.129 255.255.255.192
R1(config-if)# end
R1# show ip route

The connected route now reads 192.168.50.128/26, and the ping from PC1 to 192.168.50.170 succeeds. 6. Fault 2, a gateway outside the host's subnet. On PC2, change only the Default Gateway to 192.168.50.200, as if it had been mistyped. Then from PC2:

C:\> ping 192.168.50.10
C:\> ping 192.168.50.170

PC1 still answers and PC3 does not. Work out why with the block-size method: PC2 is 192.168.50.20/25, in the block 0 to 127, and 192.168.50.200 is in the next block, so PC2 cannot reach its gateway directly. Local traffic works; everything off the subnet fails. Put the gateway back to 192.168.50.1 and ping 192.168.50.170 again: it now replies. 7. Watch IOS refuse two bad addresses. On the unused G0/0/2, try an address inside LAN A's subnet, then the network address of a /30 from the spare block:

R1# configure terminal
R1(config)# interface g0/0/2
R1(config-if)# no shutdown
R1(config-if)# ip address 192.168.50.100 255.255.255.128
% 192.168.50.0 overlaps with GigabitEthernet0/0/0
R1(config-if)# ip address 192.168.50.192 255.255.255.252
Bad mask /30 for address 192.168.50.192
R1(config-if)# end

The first is refused because 192.168.50.0/25 already lives on G0/0/0. The second is refused because, in a /30 with a block of 4, .192 is the network address; the usable pair would be .193 and .194, and .195 is the broadcast. Neither line is stored, so G0/0/2 stays unassigned. 8. Finish with the record a colleague would want: from show ip interface g0/0/0 and show ip interface g0/0/1, write each subnet's network address, broadcast address and number of usable hosts next to your step 1 plan, and confirm they match.