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[5]MCDOWELL, Mindi and HOUSEHOLDER, Allen. Security Tip: Understanding Firewalls. 2009. Disponível em https://www.us-cert.gov/ncas/tips/ST04-004. Acesso em: 14 de maio de 2015.
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[9] BELLOVIN, S.m.; CHESWICK, W.r.. Network firewalls. Ieee Communications Magazine, [s.l.], v. 32, n. 9, p.50-57, set. 1994. Institute of Electrical & Electronics Engineers (IEEE). DOI: 10.1109/35.312843. Disponível em http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=312843&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel1%2F35%2F7577%2F00312843. Acesso em: 23 de maio de 2015