net: add examples for IPv4, ParseCIDR & IPv4Mask
net: add examples for IPv4, ParseCIDR & IPv4Mask

Further examples to support the net package.

See issue #5757

Change-Id: I839fd97a468c8d9195e8f4a0ee886ba50ca3f382
Reviewed-on: https://go-review.googlesource.com/42912
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

vsekhar/elastic-go,codestation/go,akutz/go,polyverse-security/go,CAFxX/go,polyverse-security/go,josharian/go,akutz/go,nawawi/go,cloudflare/go,codestation/go,akutz/go,golang/go,akutz/go,akutz/go,polyverse-security/go,golang/go,josharian/go,Cofyc/go,josharian/go,codestation/go,nawawi/go,polyverse-security/go,derekmarcotte/go,derekmarcotte/go,vsekhar/elastic-go,codestation/go,nawawi/go,polyverse-security/go,ImJasonH/go,christopher-henderson/Go,Cofyc/go,CAFxX/go,ImJasonH/go,golang/go,polyverse-security/go,cloudflare/go,golang/go,cloudflare/go,polyverse-security/go,josharian/go,Cofyc/go,nawawi/go,nawawi/go,josharian/go,ImJasonH/go,polyverse-security/go,josharian/go,golang/go,Cofyc/go,cloudflare/go,akutz/go,derekmarcotte/go,Cofyc/go,golang/go,josharian/go,derekmarcotte/go,ImJasonH/go,vsekhar/elastic-go,ImJasonH/go,cloudflare/go,CAFxX/go,Cofyc/go,golang/go,cloudflare/go,codestation/go,akutz/go,codestation/go,vsekhar/elastic-go,codestation/go,ImJasonH/go,ImJasonH/go,cloudflare/go,CAFxX/go,derekmarcotte/go,vsekhar/elastic-go,derekmarcotte/go,Cofyc/go,CAFxX/go,vsekhar/elastic-go,akutz/go,derekmarcotte/go,derekmarcotte/go,nawawi/go,vsekhar/elastic-go,josharian/go,CAFxX/go,nawawi/go,vsekhar/elastic-go,golang/go,codestation/go,nawawi/go,CAFxX/go,cloudflare/go,ImJasonH/go,CAFxX/go,Cofyc/go