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This is my router! the topology is one router connected to one pc on g0/1, on g0/0 its connected to a switch then connected to a pc. the whole network is 202.101.99.0/24, using VLSM we had to break it down first with 98 usable hosts so I decided for the IP address of the router for g0/0 I want to use 202.101.99.129 255.255.255.128., then I assigned the pc 202.101.99.130, I went to assign the IP address to the g0/0 and its saying theyre over lapping..why is this 

bye>    
bye>enable    
bye#config t     
bye(config)#interface g0/1    
bye(config-if)#ip address 202.101.99.129 255.255.255.128    
bye(config-if)#no shutdown    
bye(config-if)#exit    
bye(config)#int g0/0    
bye(config-if)#ip address 202.101.99.161 255.255.255.224     
% 202.101.99.160 overlaps with GigabitEthernet0/1    
bye(config-if)#ip address 202.101.99.193 255.255.255.224    
% 202.101.99.192 overlaps with GigabitEthernet0/1   
bye(config-if)#ip address 202.101.99.163 255.255.255.128    
% 202.101.99.128 overlaps with GigabitEthernet0/1    
bye(config-if)#
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  • btw for g0/0 I was using ip 202.101.99.161 255.255.255.224
    – user969645
    Commented Dec 4, 2018 at 2:17
  • This two-part answer explains how to do this and where you went wrong.
    – Ron Maupin
    Commented Dec 4, 2018 at 2:43

1 Answer 1

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For interface g0/1 you're using the network 202.101.99.128 /25. With this, the network address is 202.101.99.128, the usable hosts range from 202.101.99.129-202.101.99.254. Your broadcast address would be 202.101.99.255.

For interface g0/0 you assigned it 202.101.99.161 which is using the 202.101.99.160 /27 network. This overlaps with your previous network. Because as stated above, the 202.101.99.128 /25 network covers the ranges .129-.254.

By looking at the chart below, you can see that your first network takes up all of the addresses in the red box, ranging from .128 - .255 In order to not have overlap, you would have to assign g0/0 one of the network addresses in the green box.

If each network is supposed to have 98 usable hosts, your best option would be to use the 202.101.99.0 /25 and 202.101.99.128 /25 networks

enter image description here

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  • TYSM, but I forgot to mention that the g0/0 has 30 usable hosts, idk if that changes anything
    – user969645
    Commented Dec 4, 2018 at 2:46
  • @user969645 it doesn't change the entire concept, but to reduce address gapping, I would assign g0/1 with the network 202.101.99.0 /25and g0/0 with the network 202.101.99.128 /27.
    – DrZoo
    Commented Dec 4, 2018 at 2:49
  • for some reason it tells me using 202.101.99.0 /25 is bad mask
    – user969645
    Commented Dec 4, 2018 at 2:57
  • @user969645 that's the network address, which would show up in the routing table on the router. Assign 202.101.99.1-.127 to interfaces and PC's on that network.
    – DrZoo
    Commented Dec 4, 2018 at 2:58
  • thank you! it makes a lot more sense now
    – user969645
    Commented Dec 4, 2018 at 3:03

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