Why is there overlap with two interfaces?

Question

I am trying to configure router0, I assigned ip 192.168.26.1 to gigabitEthernet 0/0 and I tried to assign this ip: 192.168.26.33 to interface serial 0/0/0 but this error appears:

192.168.26.0 overlaps with GigabitEthernet0/0

Serial0/0/0: incorrect IP address assignment

Router(config)#interface gigabitEthernet 0/0
Router(config-if)#ip add
Router(config-if)#ip address 192.168.26.1 255.255.255.0
Router(config-if)#no sh
Router(config-if)#no shutdown 

Router(config-if)#
%LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up

Router(config-if)#exit
Router(config)#inter
Router(config)#interface ser
Router(config)#interface serial 0/0/0
Router(config-if)#ip add
Router(config-if)#ip address 192.168.26.33 255.255.255.0
% 192.168.26.0 overlaps with GigabitEthernet0/0
Router(config-if)#no sh
Router(config-if)#no shutdown 
% 192.168.26.0 overlaps with GigabitEthernet0/0
Serial0/0/0: incorrect IP address assignment

this is my subnet information:

this is my topology:

Answer 1

You can't have two router interfaces in the same subnet.

192.168.26.1/24 (255.255.255.0) and 192.168.26.33/24 are part of the same subnet. According to your diagram, you want to use a /27 mask, or 255.255.255.224.