You may edit /etc/yum.repos.d/NethServer.repo, comment out mirrorlist and add your own baseurl:
#mirrorlist=http://mirrorlist.nethserver.org/?release=$releasever&repo=base&arch=$basearch&nsrelease=$nsrelease
#baseurl=http://mirror.nethserver.org/nethserver/$releasever/base/$basearch/
baseurl=http://neth.local.domain.or.ip/nethserver/$releasever/base/$basearch/