my sysctl.conf:

---
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
---

after booting:

---
# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group
default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: wlp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group
default qlen 1000
    link/ether e4:a4:71:15:26:71 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.237/24 brd 192.168.88.255 scope global dynamic noprefixroute
wlp2s0
       valid_lft 407sec preferred_lft 407sec
    inet6 fe80::d74b:c12f:6ad7:879b/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever
---


after running command 'sysctl -p' kernel parameters are set vith values from
sysctl.conf


---

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group
default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: wlp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group
default qlen 1000
    link/ether e4:a4:71:15:26:71 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.237/24 brd 192.168.88.255 scope global dynamic noprefixroute
wlp2s0
       valid_lft 548sec preferred_lft 548sec
---